If you are on tight budget, just try looking for cheaper alternatives in places like This Fashion. Get a simple tube dress will do (not those cotton kind), invest in a good pair of shoes (say $40+/-), do your make up yourself and hair in those neighborhood home-based hair salon. You should be able to cap in within $100-150.
Dress doesn't have to be that expensive, I mean not like you gonna wear it very often after your prom, rather you invest it on a good shoes and good bag / clutch or whatever accessories you might need. You can always try borrowing instead of buying.
I didn't even spend more than $60 for my Dinner & Dance in Poly.