There is no set limit on the amount of money you need to pay Yojimbo to see Zanmato.

The attack that he does depends on many variables, the most important of which is his 'affection' level with you (the starting value depends on which version of FFX you have and which response you gave Yojimbo when you acquired him at the Cavern of the Stolen Fayth).

Then there are other factors such as his motivation level (this is where gil comes in), whether he has a full overdrive bar, and what Zanmato resistence level the fiends you're fighting have.

Most of this is probably going over your head but I posted more in depth about how the system works in a previous thread, do a search. Also there is a very in-depth FAQ at GameFAQs that explains the exact mechanics of it all.