Also by your logic, a gatherer can charge 120 million gil because they're the only ones who can collect the catalyst. Which may or may not take a lot of time to do, so they could end up doing less than the crafters and battlers. Also the gear that battlers may get in the dungeons may be way better than any thing the crafters can make, so why not let them have a little power? Most of their money is probably going to come from the materia system anyway.