Building a marketplace software budget from verified prices and the cost of the current route

A defensible budget starts with the work, not a subscription headline. This guide uses one fictional merchant established in England, its authorised seller entity, one named marketplace account and a bounded exchange: approved product records and stock move from the seller's source system; later order events return to it.

No supplier price, wage, volume, saving or return is supplied. Research closed on 6 September 2026. Every monetary cell stays blank until the buyer attaches a current record showing the legal supplier or internal source, currency, VAT status, unit, billing period, quantity, term, limits, inclusions, exclusions, assumptions and evidence date.

Fix the cost object

The budgeted object is the complete job over the buyer's chosen review period. It is not just software access. Name the seller legal entity, product population, marketplace account, source system, service boundary and accountable owner. Keep catalogue, inventory, repricing, fulfilment, payments, returns, customer service, tax or accounting, and advertising as separate functions even if one contract mentions several.

Define the starting and ending state for each record. An order placed is not automatically accepted, authorised, captured, fulfilled, retained or settled. Cancellation, return, partial or full refund, dispute and correction also create different work and evidence. Costing a vague unit called "order" hides where charges, staff time and risk arise.

HMRC's digital-platform seller guidance separates sellers from platform operators and says platform reports do not replace normal business or tax records. An accountant must reconcile the actual marketplace, payment and ledger events.

Record a price before using it

A usable external amount needs one price-evidence card:

Field	Required entry
Supplier and service	[legal entity, exact edition and UK availability]
Amount and currency	[blank until current evidence exists]
VAT status	[included, excluded, reverse charge, outside scope or unresolved after review]
Charging unit	[account, user, product, listing, event, order, revenue share or other defined meter]
Time and commitment	[billing period, minimum term, renewal and notice]
Conditions	[volume, marketplace, payment geography, eligibility and promotion]
Adjustments	[refund, dispute, failed event, overage and credit treatment]
Evidence	[first-party record, access date, version and owner]

Do not carry a web price into the budget if the contract edition or VAT basis differs. An amount described as free can still require hosting, support, extensions, internal work and exit. A transaction-linked charge needs the exact triggering event and treatment of reversals.

Cost the current route as well

The manual or existing-account process is the baseline option. Record staff activity from actual time evidence, not an invented hourly rate. For each task, collect role, event, time unit, observed quantity, payroll or contractor source, employment overhead treatment, period and confidence. Keep paid cash from the economic value of internal capacity.

The 2026 HM Treasury Green Book is public-sector appraisal guidance, not a private merchant pricing rule. Its useful methodological boundary is to compare options over their life, retain business as usual and state uncertainty. The buyer's accountant and evaluator must adapt any principle rather than claiming formal Green Book compliance.

Build the whole-life ledger

Use distinct rows for one-off and recurring costs. A practical ledger covers:

• discovery, process mapping, data clean-up and product evidence;
• configuration, interface work, migration and fictional fixture creation;
• supplier charges, hosting, extensions and managed labour;
• product, consumer, privacy, security, accessibility, contract and tax review;
• training, access administration, support and reconciliation;
• monitoring, changes, failed events, corrections and product withdrawal;
• cancellations, returns, refunds, disputes and customer support effects; and
• archive, export, isolated restore, deletion, transition and termination.

Each row needs currency, VAT treatment, unit, quantity, period, source, date, owner, confidence and scenario rule. Separate committed, incurred, forecast and contingent amounts. Do not net a possible benefit against cost before both have their own evidence.

Keep consumer and safety work visible

The cost of correct offer presentation belongs in scope. GOV.UK's online-selling page lists description, price, delivery, ordering, correction and confirmation matters. Mandatory fees, taxes and charges sit within the CMA's current price-transparency guidance. Consumer counsel must apply those sources to the actual offer.

Optional choices need their own test and record. GOV.UK says additional online charges require express consent. Budget for designing, reviewing and retesting the choice rather than treating a software toggle as proof.

For goods, include product-identity, traceability, warning, instruction, corrective-action and withdrawal work. The OPSS product-safety advice keeps Great Britain and Northern Ireland treatment separate and warns that a product-specific regime may take priority. Safety review is a gate, not an overhead to drop when the budget is tight.

Add privacy, security and accessibility costs

Map personal-data purposes, roles, subprocessors, transfers, retention, rights support and deletion. The ICO's controller-processor contract guidance is visibly under review following DUAA changes. Include qualified review and a publication-day recheck rather than freezing the page as a permanent answer.

Cookies, pixels, SDKs and similar operations need separate PECR analysis. The final ICO storage and access technologies guidance is dated 29 April 2026. Consent work, data mapping and deletion tests are not included merely because the supplier processes orders.

Security costs include supplier assurance, account configuration, least privilege, logs, incident response, protected copies, recovery and access removal. NCSC's supplier-assurance questions cover continuity and contract exit, while its SaaS guidance addresses customer-side configuration. Neither record certifies a service.

Set aside work for staff and customer accessibility. The government service-provider guide describes the Great Britain Equality Act context and the separate position in Northern Ireland. Testing must cover the actual product, price, correction, cancellation and support tasks.

Reconcile VAT without guessing

Supplier VAT treatment and the merchant's VAT on marketplace transactions are separate questions. HMRC's VAT record guidance names record categories such as supplies, adjustments, invoices and credit or debit notes. Its online-marketplace seller guidance says treatment can depend on establishment, goods location and transaction facts.

Show gross and net amounts only after the accountant confirms the basis. Keep VAT payable or recoverable apart from economic cost, and preserve the audit trail for refunds and disputes. This article provides no tax advice.

Use blank formulas

The whole-life cash budget can be expressed without pretending to know values:

Total cash requirement = setup cash + recurring supplier cash + external assurance cash + incident and exit cash + VAT cash effect [reviewed]

The economic view adds internal capacity and opportunity cost:

Whole-life economic cost = total cash cost excluding transfers as reviewed + internal labour cost + displaced-work cost + risk-adjusted recovery cost

For every symbol, record unit, period, population, source and uncertainty. Run a low, central and high buyer scenario only after the inputs have evidence. Sensitivity should show which blank or assumption changes the decision, not create a false forecast.

Separate ROI from reporting

Orders, attributed revenue and supplier dashboards do not prove incremental benefit. Start with an observed baseline for the same eligible products, account, period and state definitions. A benefit requires a credible counterfactual and must use net contribution after refunds, disputes, variable fulfilment and other relevant costs.

The HM Treasury QPIE guidance is aimed at policy evaluation, but its distinction between outcome change and attribution is a useful methodological warning. A qualified evaluator should choose a proportionate design and state contamination, missingness and uncertainty. Do not call a gross sales change ROI.

Possible avoided rework, prevented loss or released capacity remains a buyer scenario until observed and causally supported. Risk reduction should show the event, exposure, consequence method and uncertainty; absence of an incident is not automatically a saving.

Set budget controls and stop conditions

Assign each ledger row an owner, approval ceiling, evidence expiry and reforecast trigger. Keep buyer thresholds blank. Reforecast when the supplier edition, charging unit, volume, marketplace interface, product population, VAT analysis or assurance scope changes.

Stop if a price lacks its unit or VAT status, if the baseline is not measured, or if a non-compensating consumer, safety, privacy, security, accessibility, tax or exit gate fails. The merchant can retain its current process instead of treating money already spent as a reason to continue.

This publication-held framework is a budgeting method, not a quotation, forecast, tax conclusion or return promise. All live values require current evidence and named review before use.