>It's that specifying the meaning of a "good job" is much more difficult when it comes to ads.
Well that's the crux of it: it's about people and people aren't input/output machines. One message (input) doesn't predict a behavior (output).
That's why I think that anyone that promises specific results in marketing, you should step away from it. I think the best stance is that they will work with you to achieve something, and "work with you" means literally that - try to figure things out with you because your brand/product is it's own thing.
>For ads do a good job means give me most customers for my budget. And the measure of that is CAC. But I've yet to come across an ad agency that bases it's compensation on CAC.
Ok, now I think I understand you...
From the moment you're dependent on a 3rd party, basing your compensation on their work it's just soaking up risk. This goes for both ways: it's the same of some marketing agency saying to a dev shop hey guys, our marketing is solid, don't worry, we will pay you in rev share until the bill is settled... why would you take this deal?
Same goes for marketing: how can they know that you have a good sales dpt. to close leads? How can they know you have a good on boarding? Is the price competitive? They could be throwing you the best ads, getting top notch potential customers, and your product just sucks. Now what? Why should the agency pay for your bad job?