I would think a 5% discount from the $150 ticket for every year you had a full Warcraft subscription (with a maximum of 4 years or 20%) would be fair. For the other useless games like Diablo or Starcraft:P, you get your discount with proof of purchase. This would be a nice and thoughtful "thank you for your continued game play" attitude.
One of the podcasts I listened to this week (sorry I don't remember which one) mentioned that Blizzard has to make money to pay for these events and pay for entertainers like Ozzy Osbourne. Now I think a couple of posts back I said Blizzard is receiving about $170,000,000 per month in subscriptions if the figure of 11.2 million subscribers per month is accurate. Please don't tell me that Blizzard needs to make money from this event. These events are just to fuel the fire of the players. Don't get me wrong I would LOVE to be there. More so for the community of gamers that it brings together, it seems very exciting. Would I pay $150, damn straight, hence why there is no discounts. With not enough tickets for these events as they are sold out immediately. I know I know, supply and demand.
The only thing I ask, lets not pretend that Blizzard needs the $150 per ticket but that people are willing to pay that amount.