Many college teams have collections of costumes that you can borrow to compete in. So ask your team if this is the case.
Also, if you're new to this and are dancing in syllabus-level events, you aren't allowed to wear costumes. Usually only the highest-level competitors at collegiate comps wear costumes, the rest wear skirts and pretty tops or party dresses.
Barring that, your best bet for getting a fancy gown for as little as possible is to make your own, or buy a used dress from someone who makes their own.
For example, for what a new gown costs, I can make myself several new dresses a year. And after I've worn a dress between 1 and 3 times, I sell it for a very low price: just my cost of materials. Let me know what size you are and what your budget is, and I'll let you know if I have anything that will fit.
If you want to try to make your own, I can point you at a web page with lots of information on how to do it.
Laura