Doesn't offering a cut of the rehab budget incentivize your partner to blow up the budget?
I am also considering best ways to go about this, and am working on a structure similar to what @Brian Ortins mentioned above, with the slight adjustment that I also want to incentivize a contractor/ partner to get the work done ahead of schedule and under budget as well.
For example... you get 10% of rehab budget if project is completed within 5% of $10,000 budget within 6 weeks of start date. For every week past the 6 week mark that project is not completed, a 2% reduction in fee will made. For every week under the 6 week mark that project is completed, a 2% increase in fee will be rewarded. For every $1000 dollars over budget, a 2% reduction in fee will be made. For every $1000 under budget, a 2% increase in fee will be rewarded. The maximum earned cut is 15%. If cut slips to below 5%, you will be removed from project, 5% fee for finished, satisfactory work completed will be paid.
Just my .02...