So I happened to come across this ultra-cheap Teen Titans costume at Amazon, which prompted me to look at the vendor offering it, which led to the discovery that they have a ton of cheap costumes if you’re willing to page through and find them. Lots and lots of options for under $15, and yes, there’s shipping, but it’s pretty cheap, too.
Also, I personally believe you cannot place a price on not having to go to a Big Box Store with an indecisive child who has trouble choosing from amongst the overpicked, overpriced offerings. I much prefer the magic of a box arriving and “Oh, look! You should totally wear this for Halloween!” Amen.