Do you have any friends who are your size? I've gone 50/50 with a friend before and bought a formal dress for us to share. We both had occasions coming up (different dates) and didn't have the cash to spend a lot of money. This worked out perfectly for us, and we both drycleaned the dress before "loaning" it to each other.
If not, check around with your friends. Someone may have a good dress lurking in their closet.