Constant powering down is a good solution as the people who'd be contributing already give out their SP to support you. It's like saying we keep 50% of the money you give us to force you to donate forever in the future. (SP compete with SP) I think it's fair that you for you to take out some money for expense from the money raised. Even if some other people would do it for the increased reputation and feel good aspect of it most people wouldn't mind paying for administrative costs if they are transparently reported.
Good luck in your venture.