One of the perennial problems that places like DoES Liverpool and hackspaces have is how to spread the cost of running the space among the community that uses it.
At DoES we went for a more traditional fixed set of levels of use and prices, but some of the hackspaces go with a "donate what you feel is fair" approach (and then often end up with a recommended amount to guide people).
Just now I was reading about the Arduino commune that John Willshire has been attending, and they've got a lovely way of defining an "amount that's reasonable, but will vary based roughly on people's individual ability to pay":
"[...] if you’re coming along regularly, you might like to make a donation to the space (of, say, the amount of money you last withdrew from a cash machine or something like that)."
I like that.