If you're still looking to pick up Grand Theft Auto IV and don't mind the Australian version, Big W is selling the game for $78. Not convinced, here's the page with tasty confirmation.
Maths not being my strongest subject, I whacked this one into the calculator. It works out to be $41 off the RRP. I'm sure there are other retailers selling it for below what Rockstar's asking, but this is the lowest I've seen.
Big W Entertainment > Grand Theft Auto IV [Big W, thanks Hal]