Certainly by eyeballing the graph I'd say that ten is about right and 100 as way too high. However, I wonder if there aren't other considerations. For example, '10 Free Levels!!!' doesn't sound so hot.

The other thing is, if you lower it at all at this point you're going to get at least a few whines in the comments.

Another approach would be to come up with a different kind of level and sell those in a pack.