From everything I've read, around $100 should cover everything. There was some mention that for slightly more than that you could buy the tux.
Don't mean to meddle here, but if he's already agreed to the suit and tie, are you sure you should push for the tux? Especially since he's doing the suit reluctantly?
It's none of my business--just a thought. I'm a man and I can pretty much predict what my reaction would be.