I've seen this implemented before on other servers.
They had terms and conditions in there that said, if you donate to get unbanned, it doesn't mean you will be unbanned, your donation is non-refundable.
If someone is horrible on the server, and deserves a perm ban, blatantly breaking all the rules, there should be no un-ban available.
I can see a more corrupt staff banning people, and denying the ban appeal with, well you can donate to get unbanned.
I would say if someone has a Temp Ban, maybe have a donate to un-ban, if they have a perm ban, not available.
Still -1