lol my boyfriends the one who wants to go to prom. and hes out of highschool. i dont really want to but its my senior year blah blah. try renting a tux or borrow from a friend.. and i would make her dinner. i personally like that better and wouldnt be offended. and about the limo, if its just you and her in the limo, tell her she can pay. you have a perfectly fine car at home and dont need one. if its a group, spit the costs. about the girls house, maybe tell her to go on ahead and fake a sickness halfway through the dance
haha. or just tell her you dont feel comfortable going. if she loves you, she should understand all these reasons. 400+ dollars is not fair to you in the least bit. i can understand buying the tickets for her, but thats about it.
good luck!
edit- just saw the limo thing.. glad that worked out!