I would tell them that if they are any good at their job, They would be confident enough to work on a percentage basis.

No win no fee. Otherwise tell them to get lost.

( To start with, they know that if you are owed Money bad enough to need a Debt collector, Your no good at collecting debts or chasing people that haven't provided the service that they have promised. So they are prepared to Add to your problems with the knowledge that you won't chase them if they don't come up with the goods )