You might just need to sweeten the deal a bit. I do agree with a base training pay, though yes it CAN be costly. Working off the $10/hr ratio, you could do something like offer to backpay and cover the training fully after working for you for 6 months. If you are correct, and you do see an influx of students, then the instructor will have payed for himself easily by then. If not, then it was an unfortunate investment, and you can cut your losses.
Either way, it IS a difficult scenario, and you don't want to pester him too much. Knowledge and presentation is key here.