Yeah, the trouble with CPA bidding is that you need to figure out how much you are willing to lose though. If you set your Google CPA at your actual CPA, then you won't make very much money. If you set it too low, you'll get crap traffic. The innards of these systems tend to be pretty complicated, so it can be hard to figure out where to bid.

Don't work for Google, but have worked with various bidding engines over the years.