There are some really great Halloween costumes out there! Admittedly, when I think about discount department store costumes I revert to the classic plastic masks for kids costumes only, but those are now seemingly a thing of the past.
There are some surprisingly detailed costumes at Kmart -not just for kids, but for adults/couples, too! All at 30% off.
In addition to these, they have TONS for under $20 (leaving some extra change for some extra candy!)