I think $20 at most. Is that reasonable?
I'd stick with Ebay for that budget, especially if it's just for a single night, not intended to be reused for future cons or something.
You can get some really nice wigs for cheap if you know what terms to search for. Try searching "(insert color here) heat resistant cosplay wig" and see what comes up. Set the search for lowest to highest price.
Check out the pictures in the description. Do they have a picture of the wig flipped inside out? Are the wefts nice and close together? You want a thick wig that won't show off the wefts through the fibers or your natural hair while you're wearing it, so look for something nice and thick.
Ignore anything that looks even remotely shiny in the pictures, don't go for it if it has "party" in the name (those are usually suuuuuper cheap kanekalon wigs). Make sure the seller has good feedback numbers. I prefer my sellers to have at least a couple thousand ratings before I buy from them.
I've gotten some really nice wigs for between $8 and $24 from Ebay, so it's definitely possible to find quality stuff there.
Just keep in mind that shipping on most things coming from China will take 2+ weeks.
Afterthought edit: Don't search for "yellow wig". Outright yellow wigs almost always look awful when worn. Look for a blond wig.