Not good on average. Depending on the locale it could be as low as $8-$10 per hr or as high as $15-$20. This also depends on the CFI's qual's and experience.
I still maintain my certificates but haven't actively instructed for over 12 yrs. Even so, I won't get into a plane to do a BFR or IFR refresher for less than $30 per hour.
The main problem is the cost and liability of the Flight School. They will charge upwards of $25-$30 per hour with the CFI seeing less than half.