A lot depends on where you are in the country, the type of instruction you do, and the type of school you are teaching at.

I taught at a large private 141 school (non-collegiate) in the DC metro area in 2000. (~18 planes in the schools fleet, ~20 instructors)

Instructor rates were as follows: (2000 rates)
Private pilot course, BFR, Introductory flights, sightseeing: $16/hr (flight time)
Instrument, IPC, Commercial courses: $17/hr
CFI, CFII, Multiengine: $18/hr