The only way to get on next week's Halo: Reach multiplayer beta is to own a copy of Halo 3: ODST. So Microsft is cunningly reducing the price.
From today, you can get Halo 3: ODST for $69.95, a whole thirty bucks off the previous RRP. And, given that many retailers were already selling ODST for considerably less than it's $99.95 RRP, you can probably find it for even less now.
The Halo: Reach multiplayer beta begins at 3am AEST, May 4. Until then, I'd recommend grabbing ODST and enjoying the excellent Firefight mode with three of your mates.