Well the trick is that it's cheap enough that almost everyone can afford it while being pretty sure that, for that amount of money, he will probably take his time to write a decent answer.
Perhaps 5 bucks would be better, but considering an hourly wage and how much time I take to write stuff myself, ten bucks is not unreasonable. Also consider that, assuming this is a business (or could be), he's taking risk because there is no subscription or recurring payment whatsoever. I imagine this being a very variable business. If it would make a business in the first place.