Should I hold them to the price?

I'd say do what is right. You KNOW that they're worth more than 20 bucks. The only reason you could justify not paying the fair price is if they decide on their end to suck it up. You WON'T get them for that price no matter how loud you yell so the only thing left to do is decide whether or not you're going to trash their feedback over this. As previously mentioned, Karma is a ***** or as another guy put it, You reap what you sow. Do unto others and all of that.
Merry Christmas