Scaling campaigns: maximum CAC and payback period

A hypothetical store wants to move from €8,000.00 to €20,000.00 of monthly ad spend. The dashboard shows a ROAS of 4.00, which looks excellent. The question is whether that jump brings more profit or less.

The data belong to no real company. The three spend levels are observed points in the case scenario: there is no invented saturation curve and no assumed conversion uplift.

The starting figures

The order of calculation

1. Contribution margin on the average order, ex-tax.
2. The maximum CAC that margin can carry.
3. The payback period at the starting point.
4. Break-even ROAS.
5. A marginal ROAS table across the three observed levels.
6. CAC and payback for each level.

Step 1 · the margin

Out of the €78.00 average order, stripping the tax leaves €64.46, which is revenue: under the Spanish accounting rules the VAT charged is not part of it, and it is neither the taxable base nor the cash collected. Taking out product, packaging, shipping, fees and the returns provision, contribution per order is €32.34, which against net revenue is 50.2%.

Step 2 · the CAC ceiling

At 1.6 orders per customer per year, accumulated contribution is €51.75. The break-even maximum CAC is €51.75, far above the current CAC of €19.50: there is €32.25 of headroom.

Hold on to that number, because it is the one nearly everyone uses as a scaling limit. We are about to see that it is not the limit that binds.

Step 3 · payback at the starting point

Monthly contribution per customer is €4.31. Recovering the current CAC takes 4.52 months against a target of

1. Is it met today?

yes, but not by much.

Step 4 · break-even ROAS

With this contribution, break-even ROAS is 2.41 on tax-inclusive revenue, and maximum ACOS is 41.5%. Computed on net contribution, the break-even ROAS used in the table below is 1.99.

Step 5 · the three levels, and what the average hides

Looking only at the average ROAS column, the story is reassuring: it falls from 4.00 to 2.81, but stays well above the break-even of 1.99. A dashboard showing that invites you to keep going.

Looking at the steps, the story changes.

• From the first level to the second: €6,000.00 of extra spend brought €17,940.00 of revenue. Marginal ROAS 2.99, above break-even, and €3,001.00 of extra profit. That step paid for itself.
• From the second to the third: the same €6,000.00 brought only €6,240.00. Marginal ROAS 1.04, below break-even, and €−2,869.22 of profit destroyed. That step does not pay.

Total profit rises from €8,045.27 to €11,046.27 and then falls to €8,177.06, while revenue keeps climbing throughout. The profit optimum sits at €14,000.00.

Step 6 · CAC and cash at each level

Here is what makes this case interesting: there are two limits and they do not coincide.

The marginal profitability limit arrives at the third level, when marginal ROAS drops below break-even.

The cash limit arrives earlier. Already at the second level CAC rises to €21.88 and payback moves to 5.07 months, past the target of

1. That step is profitable and

does break the cash target.

And the break-even maximum CAC, €51.75, is never reached: the highest CAC of the three is €27.78. Had you used that ceiling as your stop signal, you would have scaled unchecked until profit was destroyed.

The decision, derived

It depends which constraint binds, and both answers are in the table:

If profit and marginal return decide, the level is €14,000.00: that is where total profit peaks and where the last step still pays for itself.

If cash decides, you stay at €8,000.00, because it is the only level that respects the 5-month payback target.

What the table rules out unambiguously is the jump to €20,000.00: it breaks both constraints at once and cuts profit by €−2,869.22 against the level below.

How much additional spend is profitable

Between €8,000.00 and €14,000.00 the extra spend is profitable: every extra euro returned 2.99 euros of revenue against the 1.99 needed.

Between €14,000.00 and €20,000.00 it stops being so.

Exactly where between those two points it stops paying cannot be known from these data. An intermediate level would have to be measured. Interpolating between two observed points would assume a shape of curve nobody has measured, and this case does not do that.

What to observe before scaling further

1. A real intermediate level. Try €14,000.00 plus a small step and measure, instead of jumping to the target.
2. Whether orders per customer change with the level. If the new audience buys less often, the CAC ceiling falls and everything moves.
3. Whether the average order changes. Scaling usually brings customers with a different basket, which changes contribution.
4. Whether the comparison is clean. A change of creative, season or product between two levels invalidates the marginal ROAS.
5. How much cash you have. A payback of 5.07 months has to be funded.

Independent checks

• Revenue ex-tax: €78.00 divided by 1.21, checked.
• Contribution per order: net revenue minus the five cost lines, added separately.
• Break-even ROAS: 1 divided by the contribution ratio, divided separately.
• Average and marginal ROAS at each level: divisions redone by hand.
• CAC at each level: spend divided by orders, and checked that the first one matches the declared current CAC.
• Payback: CAC divided by monthly contribution per customer.

Case assumptions

• The three spend-and-orders pairs are declared observations of the scenario, not a function.
• Contribution per order is assumed constant across levels. In reality it can move with product and customer mix.
• Orders per customer per year are assumed equal across the three levels.
• No seasonality and no delayed brand effect.

Limitations

• The case does not predict the result of an unobserved spend level.
• Attribution of revenue to spend is taken at face value; in practice it is approximate.
• Payback ignores, by construction, customer churn and the time value of money.
• Maximum CAC depends on a repeat-purchase forecast drawn from the past.
• One channel only. With several, the analysis has to be done per channel.

Sources and methodology

Sources

• Accounting rule. Spain's Plan General de Contabilidad, Royal Decree 1514/2007, recognition and measurement rule 12, consolidated text in the Boletín Oficial del Estado (BOE-A-2007-19884). It states in terms that VAT charged to customers does not form part of the revenue of the taxed transactions. This is what supports doing the whole calculation on tax-exclusive revenue. Territory: Spain. This is the Spanish accounting framework and it does not carry over to other markets on its own; the same principle exists elsewhere, but you have to open your own market's rules. In force; consolidated text consulted 9 September 2026.
• Regulator's interpretation. Ruling 5 in BOICAC 96, issued by Spain's Accounting and Audit Institute (ICAC). It confirms that taxes the company must charge on to third parties, and amounts received on behalf of third parties, do not form part of revenue. Territory: Spain. Consulted 9 September 2026.
• Tax statute. Ley 37/1992, the Spanish VAT Act, consolidated text in the Boletín Oficial del Estado (BOE-A-1992-28740). Article 88.Uno. Used only for the obligation to pass the tax on, the way it is passed on, and the relationship between the taxable person and the customer. It is not used as the basis for the accounting treatment of revenue — that is what the accounting rule above is for. Territory: Spain. In force; consolidated text consulted 9 September 2026.

What is not cited. There is no "good" payback benchmark for ecommerce here: the research turned up contradictory medians, all of them about subscription software. The 5-month target in this case is a decision of the hypothetical business, not an industry reference.

Methodology

Four Profyza calculation engines: contribution margin, maximum CAC, CAC payback and break-even ROAS; payback is additionally run once per spend level. The marginal ROAS table is generated by a private, reproducible function from the calculated margin and the three declared levels. No figure is typed by hand and all of them are checked against an independent calculation.

Currency and market: euros, European Union. The statute cited is Spanish.

Last reviewed: 9 September 2026.

Author: Profyza editorial team.

How the figures were produced. Each one is generated by the Profyza calculation engine for that step, using the inputs declared above. Every figure is also cross-checked against an independent arithmetic check — also automated — that recomputes it without calling the engine, which is what catches the text and the calculation drifting apart. It is not a human review, and this piece has not been reviewed by an economist, an accountant or a licensed adviser. If your situation needs that, get it.

Methodology: how all of this is calculated is set out on the methodology page.

Automated mathematical verification: Profyza calculation engines. This is not a human or professional review.