It's relatively simple: build a Faraday cage around the card, no EMF gets in, so no reading. In English this means that putting some sort of iron enclosure around the card should keep the electromagnetic waves from reading the card. As for which ones do this, I think it's more challenging in leather than the more
rigid "anti-wallets". I don't think price really plays into it.
Is it a "gimmick"? IDK, I mean I would expect it to work fairly well, with the right design, but I don't think that people using RFID readers to steal your credit card info is all that likely to begin with. Maybe it's more an issue in crowded cities with lots of people walking around?