I'd be interested in this too. Everywhere I find its around $40, which is too much IMO.
On a side note Wal Mart lists Burning Crusade on their website for $30. However, my local Wal Mart does have a Burning Crusade "slim pack" (which is just smaller box as far as I can tell?) which is not listed on their website and only costs $20. They didn't have any copies of Lich King so I'm not sure if something similar exists but it might be worth checking. I may try a different Wal Mart tomorrow to see if they have it and post back.
