Wow... I've been married to a sailor for 25 years, and I have never been asked this question before. In fact, I've never been to a ball, so I've never bought a dress for one. Clash of cultures, there, I'm thinking... between the ranks?
The Army has balls all the time for officers and enlisted- maybe that's the difference?
JC Penny is a good place to find nice dresses on sale... also check out places like Ross and Marshalls you can find really good deals.
the best time to shop is after the prom season when all the formals go on sale