As an FTO we don't ask anyone to stump up in advance more than they are going to use that day. In fact, we normally just ask them to settle at the end of the day before going out the door - and if they don't they don't fly that next day until their account is all square.
We never ask for more than £200 deposit for a course - enough to stop people backing out of a booking at the last minute.
That seems fair to me and I would be very wary of paying much up front - a mate of mine lost £35,000 when SECOAT went belly up (and had to work in Saudi for 12 months to earn enough cash to complete his CPL!).