I charge $20/hour to do flats. It's cheaper if its part of package deal (Where I'm doing the lineart and the colors, then flats is cheaper and its a flat rate price per page rather than per hour).
I've also hired out to have someone else do my flats, and the compensation was similar. I've had good flats artists and TERRIBLE flats artists. Having someone do a sucky job just costs me even more time to fix stuff.
You get what you pay for I guess. 