Sprint would raise prices if it gets iPhone because every other carrier's been able to do it successfully, and that user base generates a lot of income. Ironically, I don't think Sprint getting iPhone will be a boost though because Sprint would have to be in line with VZW on 3G and LTE for it to make sense to come to Sprint for iPhone.
Do iPhoners use more data? According to some reports yes. To some reports no. I think the bottomline is iPhone on another network lacking 4G would make no sense. I also hardly think Apple would have a Wimax capable iPhone now so the compelling reason for iPhoners to move to Sprint just doesn't seem to be there. Take away the prospect of "unlimited" and Sprint would gain not much in the way of subscriber base.
Not gaining subscribers wouldn't be all bad if it was a way for Sprint to begin charging more. I hate that prospect as a current subscriber, but I understand it as a stockholder.