I guess it depends on a lot of factors, but the collections agencies I've worked with take a lot more than 10%. But you pay nothing if they are unable to collect. So if you were ready to give up on a client and write off the debt anyway, there's little to lose.

At my old company, we hired a stern-voiced hourly contractor to call clients with outstanding debts. She much more than paid for herself. Some invoices she couldn't collect on we sent to an agency, but honestly at that point they were quite old and the agency had a pretty low success rate.