What is appropriate compensation for faculty mentors in an REU program?
The NSF's guidelines are that you can generally pay one faculty-month of salary.  In principle, you can propose to divide this money up however you like, but if you try to divide up the compensation among all the faculty members involved, it may not be enough money going to any single person to really be worthwhile.

As the PI of an REU site, I do not pay the other faculty supervisors anything.  However, my field is not mathematics but physics.  The faculty mentors, most of whom work in experiment but some of whom are theorists, would all be on campus doing research even if the REU students were not present.  Most of them are getting paid summer salary off other research grants.  The only direct faculty compensation goes to me, since I manage all the advertising, admissions, and logistics for the program, which really do amount to about a month of full time work.  Some other programs have a PI and co-PI who share the responsibilities and compensation basically equally, but I would suggest not dividing the money up too finely.