You grossly underestimate how conservative and risk averse most clients are.
As a consultant I have tried a variation on your approach. I've offered to work for a client for 2-3 weeks. At the end of which THEY get to decide how much I'm worth. I reserve the right not to carry on working for them but for those 2-3 weeks I will accept whatever they pay including nothing.
In over a decade not one client has accepted this deal.