delericho
Legend
The reason it could never work is because the amount of prep required increases the time, but the 'customer' is only getting a few hours' entertainment... A kid's magician at birthday parties may have spent months perfecting his act, but he only gets paid for a couple of hours' work; you don't pay him for 8 months.
Yep, that's exactly the issue. As far as I can see, the only real way to make it work is as I mentioned up-thread, or as you mention here with your "kid's magician" example - the GM prepares his 'act' ahead of time and then runs that same adventure with many different groups. Though 'hundreds' is probably a bit extreme, since neither the training nor the material requirements are as great for the GM as they are for the magician/musician/whatever. Still, good luck finding even 3 groups willing to pay £100+ a time to play through your adventure.