In Ohio Kaplan pays $15/hour for actual teaching/tutoring, and $7 for non-teaching (preparing in advance, proctoring, training).
NOTE that the pay is the SAME for any kind of class. So, you can bust your butt on Organic Chemistry and earn $15/hour, or you can walk some high school kids through basic algebra and earn... $15/hour. It didn't take me long to realize that the SAT/ACT stuff is a heckuva lot easier than quantum physics.
If your Center has a lot of MCAT teachers, then you probably won't get many classes and they may deliberately not have you teach MCAT until you've taught lower level classes. If you're next to a gigantic undergrad school like Arizona State or something, expect to pick up a lot of hours fast.