As long as it’s conservative where it needs to be conservative it should be fine.
What branch of the service is your date? Is he Navy and will be wearing dress blues? Dress whites? Maybe what he wears will influence you a little, but it doesn’t have to.
A dark color like Navy or black, can be a very simple dress, and be formal enough. I mention it, because simple often is less expensive.
Macy’s was in last call a week ago if you do want to try the mall. I bought $90 pants for $18. You have to find the clearance section. They might have already shipped out what’s left, I’m not sure.
Is the ball very soon? Prom
Dresses will be going on sale soon. That’s another possible mall option. It still won’t be as cheap as a second hand store probably, but could be reasonable. Dillard’s usually has a great Prom dress section, very reasonable, and I did walk by that section in Macy’s, and the dresses out were very nice. I don’t know how inexpensive you need, but plenty of dresses under $100 right now. I don’t know if places like Kohls have fancy dresses? That might be cheaper.
You could ask some girlfriends if they have a dress you can borrow.
What about your date? Will he help you buy the dress?