I guess any bid has to be realistic, and in line with the standards of the day. We are currently between The Olympics and The Paralympics, where there were 70,000 volunteers to drive down costs (called "Games Makers") of running the London 2012 Games. And we assume that the London 2012 bid was funded.....
http://en.wikipedia.org/wiki/London_bid_for_the_2012_Summer_Olympics
As for Wikimania bid costs (that is up to the bid being won) expenses would be a good start.
And that other figure (of around £40K) perhaps could be a "no win, no fee" deal?
Gordo