It would be smart to take a portion of the business and take a monthly cut of the profits, but then you still need to be part of that business and have some kind of control to it. I would just negotiate with the new owner for a price that you can both live with... that's the only way that makes sense really, make sure you're both happy with it.
If you want to cut and run and take a profit, you could just sell the business to him for a price you can live with.
There's not a ton of math to be done here really if he is on a certain budget that can't surpass, so it would really just be best to sit down and come up with a number.