Surprisingly I've had good results with Google's CPA bidding option. If anything, I might overbid on CPA to drive volume, which leads to the target CPA I'm really after. It's not perfect, but it's free and having used pay-for tools, I've seen it out-perform them. You'll have to use their conversion tracking though, and probably wait for a good month of stats before being able to use it.
If you're not comfortable with that, you might consider using AdWords Editor to filter keywords weekly according to performance buckets (example: Under target CPA & not in top positions, Over target CPA, Way over target CPA). Then adjust bids in bulk by a percentage, according to bucket. Depending on the size of your campaigns you may need to do this more/less frequently.
None of this is as granular as running off of actual revenue per click, but it's a decent alternative if you don't have the budget for a tool.