Our Party City has a row of clearance year round and there's usually always some children's costumes on that row. There were some Disney costumes when we last checked a couple of weeks ago, and I think that they were all about $9.99 and under. And also, our local
Disney Store has some of the halloween princess dresses left over and some of their spring line of dress up dresses and they're all $14.99 and under now. We just got a christmas angel a Cinderella costume for $9.99 and the shoes for $4.99.
Also, this is more of a thing for next year if she's still into dress up, but hit up
Walmart the day after halloween and continue going throughout that week. Costumes go to 50% off that first day after and continue to go up to about 75%-90% off and you can get some AMAZING deals on costumes. I used a bin I already had and purchased about $20 in costumes for my nephew, and it bought at least 12 different costumes for him.
I also recommend if you or family/friends have any dresses you don't want anymore, maybe hand them down to your daughter? It's free and you can cut off length or remove sleeves or do whatever you'd like to them, and I know as a kid as much as I loved my storebought costumes my favorite were outfits of my mom's, and walking around the house in her heels.
If you have a Ross near you, they generally have costumes year round in their kids department, although it's not a very big selection. But you can usually find good deals on them there, no matter what the season.