This seems (to me) like an obvious suggestion, so my apologies if it's already been thought up.
Why not establish a team/scoreboard system, like those used for distributed computing and BitCoin mining? This elegantly solves a few problems while with minimal resource commitment from the Tor organization.
Most importantly, it's a way to pump money into the system without ruining the current atmosphere. Sponsors could easily buy some bandwidth or people can also just donate their own connections and join a particular team. The scoreboard is based on goodwill, not dollars spent.
It also eliminates the hassle of setting prices, as teams can compete for dollars and bandwidth provided, essentially setting their own prices. The org could also setup a payment system, like we-pay, that the team admins can configure to deposit funds. It could be set as a proof of work system, paying after the bandwidth has been provided.
This also allows a degree of control to prevent abuse from admins trying to juice stats by abusing some network infrastructure, like dummy trial accounts on hosting sites. A group admin could block specific hosts or the Tor project could remove an entire group.
Finally, this could allow for the Tor project to create metrics based on things other than speed, akin to how Folding@home scores GPU and CPU contributions differently. Then the project can set anonymity goals for the network (such as location, ISP, backbone provider, etc) and the volunteers will adjust their patterns accordingly.
Finally, I would like to strongly suggest taking a cut of all donations to pay for the new infrastructure and Tor research and development. I wouldn't want development to slow on Tor because donations are going to bandwidth instead. Indeed, I would prefer we spend a ton of money on the stenography efforts and usability.
Anyway, thanks for the hard work! -Zach Lym