Modelling subscription commerce return on investment in pounds

What to take away

Build the return on investment model

Start from contribution margin, not revenue

Revenue is the wrong start: take the monthly price and subtract payment processing, fulfilment, support and any third-party platform fee, which gives contribution margin per subscriber.

A £30 plan with £11 of variable costs leaves £19; every later figure uses £19, not £30. Teams that skip this get a healthy-looking return until finance reviews it. Work through the full subscription commerce costs and budget guide for England before building the model.

Recalculate margin whenever a platform or payment fee changes. A one-point fee shift moves every downstream figure, including payback.

Plot retention as a curve, not an average

A single average customer life hides the shape of churn. Build a monthly curve: how many of 1,000 sign-ups remain at month 1, 3, 6, 12 and 24. Early churn usually falls steeply, then flattens.

Multiply each month's surviving subscribers by contribution margin, then add the months. That total is lifetime contribution margin per cohort. Compare it with the cost of acquiring that cohort. If acquisition cost is £45 per subscriber and lifetime contribution margin is £120, the cohort returns 2.7 times its cost before overheads.

Compare two cohorts bought at different prices or through different channels. The gap shows how much your acquisition mix shapes the return.

Add the renewal and compliance costs

Renewal reminders, pre-contract information and cancellation handling cost money. Add staff time and tooling cost per subscriber per year, then re-run the curve.

Compliance obligations under the Digital Markets, Competition and Consumers Act 2024 sit inside this line. Treat them as a permanent cost, not a launch cost. General duties for businesses appear in the legislation and regulation guidance, worth reading alongside your model.

Turn the model into a decision

Calculate payback and a labelled ROI

Payback is the month when cumulative contribution margin covers acquisition cost; with £19 margin and £45 acquisition cost, payback lands in month 3.

ROI is total contribution margin over the modelled period minus total cost, divided by total cost; label it clearly. State the period and whether the figure is gross or net of overheads, for example a 24-month ROI of 140 per cent on a £60,000 programme. ROI without a stated horizon is meaningless.

Stress-test the assumptions

Change one variable at a time: price down 10 per cent, churn up 20 per cent, acquisition cost up £10, and watch which change breaks payback.

If a small churn rise pushes payback past 18 months, the model is fragile. Keep the product's data collection limited to what the model needs. Follow the ICO guidance on designing products that protect privacy, because extra data collection adds cost without improving return.

Common questions

Should return on investment use revenue or margin?

Use contribution margin. Revenue ignores the variable costs that rise with every subscriber. It overstates the return and will not survive a finance review.

How long a period should the model cover?

Long enough to pass payback and reach the flat part of the retention curve. For most subscription products that means 24 to 36 months, stated explicitly.

What costs do teams usually forget?

Renewal reminders, cancellation handling, support during the first 90 days and platform fees. Our guide to subscription commerce hidden costs in England lists the ones that most often sit outside the first draft.

Glossary