Hi Elizabeth,
I agree with others who have said around 50% would be more incentive for affiliates to join and use stronger marketing techniques.
I'm just down here looking for help in beginning my very first affiliate program...what I need, etc, and ran across your question. I'd like to mention a couple things which you may be overlooking.
Earlier you had mentioned that you could make $$$ rather than $ if the sales had been your own rather than through affiliates. Please keep in mind that just because you're having affiliates promote your product does not mean you cannot make sales directly yourself as well.
Also, when affiliates "pre-sell" you and your product to their readers, you are suddenly in a position to turn these people into YOUR customers for a long time to come...which can be far more valuable than an initial sale on one awesome item. I would suggest pre-planning how you will follow up with these customers to gain their trust in you and your products. It's kind of like being asked, "would you like fries with that?", when ordering a Big Mac.
Perhaps you could surprise buyers with a bonus they aren't expecting - just for ordering. This could be a free opt-in six week informative "newsletter" providing brief and helpful suggestions in conjunction with your product...maybe including college financing options, etc(?) I'm not sure what would appeal to your niche but I'm sure that you already do know.
I think the goal is not only to provide the one awesome thing you have to sell them, but let them know you have a sincere interest in seeing them achieve their goal with your product, keep them interested, keep them gaining trust in you and keep them in tow for promoting your future products to.
I have a lot to learn and may be wrong, but at least, that's what I *think* would bring some long-term following and recurring sales for you.
PS: I just wanted to add that as an affiliate it has been my experience that "good" affiliates will often purchase a product themselves before promoting it. It is my opinion that, if your affiliates are pleased, even THEY will seek out your future products either to use for themselves, or to promote for you, or both.