To run the test, take what an average settlement earns you and multiply it by the share of leads you close. That is the most a lead can cost before it loses money. Compare a supplier's price per lead against it. Then compare the alternatives on the same basis: a retainer paid whether or not leads arrive, or your own hours spent on ads and follow up.
What moves our price is whether the lead is exclusive, what was checked before delivery, the loan purposes and states you want, and the volume you start at. It is agreed as a fixed cost per lead before the campaign runs, with no retainer and no lock in, which is what makes the test cheap. The model is set out under pay per lead in Australia.