If they give you a number, then you say "Put it in a MOU where you will pay us 10% of that number if these discussions break off for ANY reason."
Only after you have a number you would actually sell the company for, and you know that if they are wasting your time you will still get enough cash to cover the extended runway you will need to keep going after they leave, then it is possible they have enough skin in the game to be aligned more with your interests.