I actually did well with my wedding dress, and bought all of my accessories (veil, tiara, etc) off of ebay.
I bought a dress for my junior prom, but for my senior prom which I did not have a date to, I wore a dress of my friend's that she had paid $12.99 for at a secondhand store--can't beat that price!!!
But go, wear the dress, do all of the fun stuff--you only go to one or two proms in your life--enjoy them