Here is what the server that I normally play on's policy:
(We have iConomy and the Chest Shop plugins, so that's what the money is for)
Donator:
~$5/month~
20k ingame
Special donator kit (60 minute cooldown)
MagicCarpet
Ability to wear blocks as hats
The Donator title in game
SuperDonator:
~$10/month~
All Donator powers
30k ingame
Special donator kit (30 minute cooldown)
Can change your display name
The SuperDonator title in game
EliteDonator:
~$25 once~
All SuperDonator powers
50k in game
Special donator kit (15 minute cooldown)
Ability to fly with your own client mod
Ability to change time
Never take fall damage
The EliteDonator title in game
UberDonator:
~$50 once~
All EliteDonator powers
75k in game
Special donator kit (10 minute cooldown)
Never lose money on death
Ability to change time/weather.
Custom rank title of your choice
The UberDonator title in game
The Donator Kit is a kit of Diamond tools (Pick, Axe, and Shovel)
This may seem like a lot, but a lot of people donate.
The best part about this is that the owners don't have to give you the powers, it's just that they trust that you are trusting enough to pay money to people you don't know in real life.
I am all for donators getting benefits, but if they misuse them they get them taken away.