There is no fixed criterion. You can see the number of dealers you have for a session and find out how much would it cost you for the venue, trainer and administrative charges. Then you can keep a margin and come up with a cost. Do some research and talk to some car dealers and ask them what would be comfortable for them.. since you have been offering the training for free, the obvious questions would be - why shall I pay for this training now. It the dealers have been gaining from your past workshops they would not mind paying some charges for them now.