BukkitExpanse is a plugin I'm writing for the upcoming 1.8 release. When fully implemented, it will allow players to use one or more currency types to expand the size of the world.
Pricing for expansion is calculated per block, with an option to include time as a factor as well. Vanilla 1.8 has a command that takes both the expansion of the radius and the time to expand as arguments, and I thought this could be an interesting thing to include in the pricing. The general idea for the formula is as follows: ((new diameter)^2 - (old diameter)^2) * (k / time)
where k is a constant which has not yet been decided. Time will probably be measured in minutes, but I suppose that could be configurable.