But it only grows with the square root of the number of wealthy individuals. See the paper with Posner; we go through many such calculations: http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2343956
A bit offtopic, but what's your (or the field's) take on buying/bribing/bullying voters?
Even if 10 000 poor person just needs to amass 10K USD, a sufficiently brazen party can just organize for 10 000 other easily mobilized people to vote against the other poor folk. (A common theme around the smaller villages here is the election day free meals, or course the place in full party colors to remind the people who to thank for.) Or is this not so common/significant in practice?)
It also seems really quite repugnant to tell people "your vote was overruled by one wealthy person, but it's ok because here's some money!" Why should being given some money automatically be assumed to make up for the fact that someone else was able to overrule your vote?