I think that's a great idea on setting the limit, as long as the person planned to drop is reasonable (by reasonable, I mean not dropping a 20M player because you don't like him anymore and he isn't quite worth that). I think if someone chooses a player on their team to drop that is 20M that is too high a number for every team to drop. If this is the plan we should just have a quick survey in here about who they want to drop with this clause (once decided we are doing it) to find the number.
That's exactly what I was thinking with regards to amount of players dropped, where someone could recreate half their team by dropping a bunch of cheap guys. 2 or 3 guys is a good limit for me as well, although if I'm picking I'm leaning at 2.