I wouldn't charge too much. Figure how much the average person pays for gas and figure how much money they would save for gas and then charge them how much they would save after 4 tanks of gas or so.

Thats just a rough idea but you want to make it fiancialy profitable for them. Whats the point of them learning how to drive if there isn't a reasonably quick pay off.

My hydroponics experiment

You ever notice how hard it is to lip read cartoon characters?
"Crazy is what the sane call Delta Flyer"