When will GL500 get discounted?

It's possible they won't. Mercedes are in a different class than Dodge, Ford, Chevy, etc. No one wakes up one day, sees an ad on TV saying "Ohhh, the '07 Mercedes are on sale now!" and has to go buy one. Most of the folks hunting these will look for them and pay the sticker price. In fact, I can't recall the last major-market Mercedes TV ad that I saw. It's kind of like asking "When will the '07 Ferraris go on sale?" Most likely not...Good luck, perhaps you can strike a deal on a certain one at a dealership looking to move some, but it most likely won't be a nationwide type thing.