Well there is lot of such sponsors - I am personally using
www.exitmoney.com .35 cent/imp
As for number of impressions counted it is not that easy. It depends on type of page the program opens. Anyway average ratio is lower than 20%. So from 100k/hits you can possibly have 15-20k counted impressions.
As you can see the same (or more money) should be made from sign-ups, but its nice additional income.
There is no easy way to make money in this business
XM