It doesn't really matter how much you pay him. If you use him wisely, eg. not letting him die, not brining him out for nothing etc, then in time the price needed to activate Zanmoto will lower. I've got it so he does for about 1000 gil, but it doens't happen everytime.
Of course, the easiest way is to save it, enter a battle, give him all your money and if he doesn't use it, reset. That way is helpful for some hard battles, such as the dark aeons
I'll decide