$50 an hour isn't too high here. It depends on what you tutor and how qualified you are. I, personally, would charge $35 an hour to tutor high school English students. But that's because I don't feel as qualified as some teachers with a Master's in English, so I don't think I could do $50 an hour.
The big draw here is high school math and science. If you can do those, you can even go above $50 an hour!
Think of it this way, though. At Sylvan Learning Centers, they charge $40-45 an hour for a kid to be one of three kids "tutored" by one person. I figure that $40 has got to be fair, then, if it's one on one!!
I'd love to hear how it goes for you... when I finally get to quit teaching, I plan to tutor as supplemental income.
And by the way, to those of you who miss teaching, anyone wanna sub for me for a few days? WHEW, I am worn out and desperately need a break!