Prom Costs... Need help ASAP!?

My boyfriend and I plan on going to Prom together. But there's a problem; I don't know how much hes paying for. I know hes actually buying our tickets but, I'm not the kind of girl that likes having so much money spent on me. And i don't want him to spend SO much. So how should i talk to him about it. Like should i pay for parts of it? If so, which parts? Like I'm sure ill pay for my part of the limo (unless he plans to :/ ) And my dress. Do i pay for dinner? I just don't know. Ive never gone to something as extravagant as this. I don't know how to go about it. HELP!!!?