Lets say it costs $3 to acquire a customer on average. And your margin is $30. That sounds good.
Now do an experiment. Stop your ads.
Do your sales go down 100%? In that cost the $3 cost was definitely justified.
Do your sales go down 50%? That means you actually paid $6 to acquire a customer, not $3. Still sounds like a reasonable deal since you still make $24 profit, but other channels might be more profitable.
Do your sales go down 10%? Then only one in 10 customers actually converted because of the ad, and you are paying $30 to acquire a customer and it would be better to stop the campaign. Google is just showing ads to people that would buy your stuff anyway. (Side note: I think this is a big problem with retargeting. You are paying to show ads to people that are already very likely to buy your stuff -- or even worse, who have bought it already. But conversion rates are really high!)
(To be fair, this is speculation and not based on experience with adwords. My only experience with adwords was a $100 campaign I made a few years back with promotional credit that resulted in no conversions)
