How to stop your Meta or Google Search customer acquisition cost (CAC) rising?

So how do you diagnose what is actually causing this and decide when it is time to expand into new channels?

This is a question we hear constantly. Many brands see early success by mastering Meta and Search. But at a certain point, often when monthly spend tips past the £100k PCM, additional budget simply drives higher CAC rather than incremental growth.

So what could be happening?

➡️Have you maxed out Meta/ Search spend?

This sounds obvious, but it is often poorly measured. The next £ of spend is not driving you the same returns as existing spend, that's your diminishing returns. The key is identifying the exact spend level where performance starts to decline, rather than assuming there is still room to scale.

➡️What is actually driving sales?

At higher spend levels, channel attribution becomes critical. Relying on GA4 last-click or blended reports is causing a cost. This is where incrementality-led measurement becomes essential, whether through Geo testing or MMM. These approaches help you understand which channels and creatives are genuinely driving incremental growth.

Once you have this clarity, you can answer a crucial question: is there still headroom in Meta or Search, or do you need to look elsewhere?

If expansion is the answer, there are typically two routes.

1️⃣Same format, new channel

The easiest decision is to follow the same approach and test across the different social platforms e.g. TikTok and YouTube.

• Creative can be reused. Low barrier entry to run and assess if it is driving acquisition.

• Likely still advertising towards a similar audience and measurement becomes tricky. A Geo testing structure would be recommended to assess impact.

2️⃣Testing offline media

Pushing into channels with wider audiences such as Linear TV or Outdoor advertising.

• Reaching new & wider audiences.

• Higher upfront costs and challenges with measurement not being available in typical reporting e.g. GA4.

A great example of this is this story from Purdy & Figg moving into testing TV & seeing it become one of the most efficient channels.

Customer acquisition costs need not be constantly increasing

The right measurement can assess what's working & expanding channels can discover more cost-effective routes.