It's true, you can get them dirt cheap because there are tons of them out there, even with the plastic still on. You'll only get a couple dollars each for them.
The only Ancient Mew that's worth a lot is the misprint one that came with the Japanese Lugia book; the misprint says "Nintedo" where it should say "Nintendo". I've wanted one for a while, but at the moment it's selling for $50 Canadian without the book and twice that with it.