There must be a balance here. Whilst no-one should ever pay huge upfront amounts, any business (especially a flight training business) has to plan ahead for aircraft, staff and instructor resources to be in place to be able to provide the training on time. I'm sure there are plenty of instances where students cancel their course last minute leaving the school in a difficult situation, so it works both ways. A few hundred pounds/dollars non-returnable deposit will not be a deterrent to anyone cancelling and some smaller modular schools could suffer serious financial consequences if they were ready for an intake of say half a dozen students in a month, to suddenly be left with half that number.
$5,000-10,000 is not an unreasonable deposit against a $50,000-$100,000 course (10%) so the school can plan ahead and get ready. My advice however, is make an arrangement with the school to place the deposit in an "Escrow' account whereby it can only be drawn down when training starts, likewise, the student cannot back out either without forfeiting. Such contractual banking arrangements can be easily made for usually a 5% fee from what I understand. Of course, with such an arrangement, the school could agree a refund or a delayed training date if the student could justify some serious life event which stops them from honoring the part of the contract, any good school would certainly do this.
SJ