DigitalAlex - 1:46 pm on Sep 8, 2010 (gmt 0)
Full disclosure: I run marketing at ClickEquations (http://www.clickequations.com)
There are 2 recent reports about paid search software that you may find useful:
If you're seriously considering one of the solutions above and percent of spend pricing is the only issue, then I would encourage you to contact the Sales department at each company. Most will be willing to negotiate a flat pricing arrangement.