A new marketing channel doesn’t have to acquire customers more cheaply than your historical average CAC to make economic sense. When deciding where to put your next marketing budget, it can be more useful to compare it with the cost of acquiring an additional customer from a channel that already works.

Average CAC and the cost of an additional customer

Let’s say Google Ads brings you customers at an average cost of €250. But across two comparable periods, the numbers looked like this:

€10,000 → 40 new customers
€12,000 → 44 new customers

The average CAC barely changed: €250 → €273.

How to calculate marginal CAC

But look only at the additional result:

+€2,000 → +4 customers

So:

Marginal CAC ≈ €2,000 ÷ 4 = €500

This doesn’t mean that your next customer from Google Ads will cost exactly €500. Seasonality, demand, competition and other factors can affect the result. But under comparable conditions, the calculation reveals something important: further growth in a channel that already works can cost considerably more than its historical average CAC suggests.

How to compare a new channel with one that already works

So if you’re considering a new source of customers — including building your company’s visibility in AI answers — the question shouldn’t only be:

“Can the new channel beat my €250 customer acquisition cost?”

A more useful question is:

“How much does it cost me today to acquire an additional customer from a channel that already works — and where should I put my next growth budget?”

AI visibility doesn’t guarantee a specific number of customers, so you can’t know the CAC of this channel in advance.

But rejecting a new channel simply because it appears more expensive than your historical average CAC can also lead to the wrong decision.

When deciding where to put your next marketing budget, compare a new growth channel not only with what customers have cost you in the past, but also with what further growth is costing you where you already are.

Sources: Google Ads Help — Optimize for marginal ROI instead of average ROI; Google — Why every CMO needs a saturation curve.

To establish a measurable starting point before investing, see the AI Visibility Audit

Want to assess the potential of your company’s AI visibility?

We start with real customer scenarios and a diagnosis of the company’s current AI visibility.

VIEW THE AUDIT