How much would you pay to double your Twitter following?

That depends, is this going to be through legitimate posts and the effort of a real team of people I can chat? I'd pay handsomely for it.

Also, are we going from 500 to 1000? or 50,000 to 100,000? I would pay more for the latter then the former, but in general, I think you can charge more if your doubling them from an already large amount vs 500 -> 1000.