Depends on how you view the sale. If you are fully transferring rights I would just sell it to them, with the price set at 100 dollars an hour for however many

Depends on how you view the sale. If you are fully transferring rights I would just sell it to them, with the price set at 100 dollars an hour for however many hours you estimate you worked on it. If you plan on retaining rights I would ask for 20-30% of that first figure plus 10-20% of revenue generated(with some stipulation on what that number is if they give it away as a value add).