orcishgamer: I knew they'd have to do that eventually, kind of hard to draw in new players if they have to spend an extra 50+ on old expacs.
What about Cataclysm? That's kind of required too... still 40 bucks or so?
Every expansion is well worth the money, but I guess the game being a MMO with 4 expansions now, Blizzard cannot stay with the traditional business model.
You can get Cataclysm for $10-$40, depending on where you buy it from. I expect Cataclysm to be released for free once an expansion after MoP is released.
EDIT: TBC was made free near the release of Catalysm, so it is possible that they'll continue this trend after MoP.
EDIT2: It appears that they also moved fully away from retail boxes, relative to their store. Only WoW, D2 BC, and SC BC are available as retail, probably leftover copies.