I don't know about codes, but I was just looking at restaurant.com because I can get those certificates for free through my employer's reward program. It's showing the $50 certificate (off $100 or more) for $20. I don't know if it's always that price or if it's because I just signed up for an account.
I guess that's not too great of a deal if you buy the cert, because you're still spending $70 for $100 of food. But if you're going there anyway...