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Is there a solution i can buy one-off, or do the better ones charge a percentage of spend?
Good place to start is Overture approved solutions (they have back end access to overture)
Why wouldn't you just focus on Google AdWords and Overture? From what we see, they account for 95-97% of the converting traffic out there; moreover, it's much easier to find a solution for those two than one that handles umpteen different PPCSE's.