I have reviewed five of the big commercial PPC mgmt firms. They mostly charge 10-15% of the monthly spend but also set a minimum figure. If you are competing with them, be prepared to give very pretty (but relevant and useful) reports.
Doesn't Gotoast do that also? It might be better to start with software and then move up to personal PPC management. It does get quite expensive. If you are spending 20,000 per month, normally, you would have to give the PPC management company small piece of the pie. If you are only spending a couple of thousand per month, then I do not believe that they will take that time of account. If they do, it would be sort of a minimal price quota. Then, you would have to wonder how they treat their smaller customers. Ask for references.