There's No Such Thing As CAC Per Channel.
The 'cost per new customer' your ad platform shows you isn't what that channel costs. It's what that channel could tie to a click of its own.
Real CAC is one number for the whole business
New customer acquisition cost is total ad spend divided by total new customers, across every channel, full stop. It doesn't exist per platform, because a customer doesn't belong to a platform. They belong to your business, and they got there by some real, often messy, multi-touch path that no single dashboard ever saw start to finish.
What your platform actually shows you is a different number wearing the same name
Meta's 'cost per new customer' is cost per first-time customer that Meta's own tracking can claim. If Meta's ad is what actually created the buyer, but that buyer converts three days later through a branded Google search, Meta's dashboard shows nothing and Google's shows a cheap, efficient new customer. Google didn't create that demand. It just happened to be standing there when the sale closed, and its CAC column takes full credit for work it didn't do.
This is why per-channel CAC arguments never resolve
Every platform is measuring customers it can prove are its own, not customers a channel actually produced, and those are structurally different numbers. The only way to see real CAC is to stop asking each platform to grade its own homework and instead reconcile every channel against your actual new-customer count, in one warehouse that isn't trying to claim credit for anything. Once that number exists, the per-channel argument mostly stops happening, because everyone's finally looking at the same math.