The optimal strategy for most businesses is one that google doesn't offer...
Most businesses have some marginal cost of providing service to a user, and are therefore happy to pay any amount up to that cost to get a user. So in the Google Ad dashboard, that would be "Target CPA", but instead of setting an average cost per acquisition, it would set the maximum cost per acquisition.
Whats the difference you ask? Some users will come via a 'cheap' route - for example directly searching for your company name, and clicking the top ad, costing you very little because your quality score is very high, and then converting at a high rate.
Other users will come via an expensive route - searching for a tangentially related keyword, unlikley to click through, and also unlikely to convert when they have clicked through.
You only want the guy from the expensive route if he, and all the others like him, cost less than your companies marginal cost of providing service. You don't want to include the guy who directly searched for your company name in the average when making that decision.
Why doesn't Google offer this? It would significantly cut down on overspend in the ad world (ie. spending more to get some customers than they are worth), and in turn significantly hurt their revenue and the internet as a whole.
3rd parties that use Googles RTB API can offer this though, although they are at a disadvantage of having a lot less private user data for targeting...
