As I mentioned in the World guard thread this would be more complicated then I thought at first, because there can be multiple owners in a region perhaps adding a creator or primary owner section.

Moved: http://redmine.sk89q.com/issues/18

Maybe it can give a quota per some time period, although I don't really like that solution.

What if WorldGuard kept track of how many cubes were claimed by each player? If a saved file maintained each claimed region by player then the cube count could be added up and checked when claim command is executed. Then when a claimed region is removed the cube count would automatically be reduced.
Or else, it might be possible to put a claimant field (aside from owner, since there can be more than one of those) in the region save file... which would be the better solution I think since the data system is already in place.

What about allowing only one owner and adding everyone else as member?

Would it be possible to integrate that with an economy, so that the claimed region would be limited by the player's currency?

I suppose, but not all servers have economy.

Would it be possible to have your plugin check if there is one and use that, or would you have to support specific economy plugins?