How much is your time worth?
There are some useful calculators (like this one: http://freelanceswitch.com/rates/ ) to help you figure out a baseline that makes sense for you. Unfortunately, it's hard to give you meaningful answers since your rates must reflect your situation.
The "going" rates for this kind of thing will be tough to track down, but depending on your experience level I would estimate these being worth about $250 per exercise to you. That's assuming your time is worth about $50/hour and it takes you 5 hours to write one — not that you'd charge hourly, but it's a useful way of estimating costs. Alternatively you could negotiate a package deal for a bunch of exercises.
This is all very subjective territory, so really the best answer to your question is this: figure out how much you have to charge in order to have the work be worth your time — and then charge that.