The whole purpose of the offer is to let the client decide how much you're worth to them. I provide no up-front rate or fee.
It's always the clients who claim to be 'innovative', 'game changing', 'disruptive' etc. that are the most risk averse. They seem completely blind to the cognitive dissonance.
Maybe it'd require a trip to their (no doubt expensive) lawyers to make sure the arrangement doesn't sound too good to be true and open them up to legal action if you're not happy. Perhaps it's not worth the time/effort/cost for them?
It's always the clients who claim to be 'innovative', 'game changing', 'disruptive' etc. that are the most risk averse. They seem completely blind to the cognitive dissonance.