Dang it, I read the question wrong. It was CPC not CPA *facepalm*. Ma bad, sorry for the confusion!
I haven't actually given the CPA option a try yet on AdWords though. Then again I've always just promoted other affiliate programs on AdWords so using the CPA model on AdWords/Facebook, is unfortunately not possible (I wish it was). Maybe it is possible if you can settle on something with the advertiser.