Goodwill is definitely a store that is hit and miss. Sometimes you find good stuff, sometimes you don't. In my case, sometimes I find good stuff but it's often overpriced in my opinion(compared to other thrift stores that are also for charity and have similar merchandise).
In the past I have gotten some costumes for my kids to either wear at Halloween or just to use as dress up. A few years ago I found a really cute Elmo costume for my son (when he was still in his Elmo phase) and it was in pretty good condition between $10-13 dollars.
However I was in a Goodwill not that long ago and was surprised at how much they had on their costumes, even ones that were missing stuff! But if you're in the area of a Goodwill, I'd still give it a try because you never know if there will be treasure mixed in all the other stuff.