There is a ton of precedence for this type of activity. My personal experience is with gaming communities. It is incredibly common in the Minecraft community. Higher donation amounts grant more and more privileges - often including game breaking abilities. A Google search for "donate for admin" shows that 8 of the first 10 results are Minecraft communities promising administration powers in return for money. The behaviour is common in other communities as well. Basically any game where a special "privilege" can be granted, it has been added to a donor perks package.
On the non-game server side, there are several plugins for popular forum software that can modify user permissions after a donation is received. Using PHPBB, for example, you can utilize this modification to reward donations. The same type of modification exists with MyBB, vBulletin, XenForo and others. These types of plugins make it easy for communities to set up special "Donor" groups with increased moderation abilities.
In the gaming community, there is a stigma associated with "paying for admin". Often this stigma is directed at both the community itself and the players that paid for the increased privileges. On the community side, this type of incentive is seen as a money grabbing scheme. Especially when higher donation amounts provide in game abilities that no one else could ever achieve. The perception is that users will want to show off to their friends and community members, thus they shell out large amounts of money for some fancy pixels. As people do this in a popular community, others see these fancy pixels and want some of their own. The cycle repeats as they go spend money. In large communities, the owners bring in more than enough to cover server costs. If they don't "reinvest" in the community with the remainder, the perception that they are doing this for the money increases.