How can Valve expect us to buy that a Quick Sequel is worth the extra money of a full release, when it's been how long since they released Half-Life 2, Episode 2? Sure, they threw-out the Lost Coast teaser, 30 seconds of game play, whooptie doo!. I suppose there's a lot less story line that goes into L4D2, but that's what made it even harder to pay full-price for. New weapons, monsters, and scenery on the same engine does not a full-release make.
I would have paid $15-$20 when L4D2 first came out if it had been an add-on to L4D. Instead I waited until it was $10 during their 4th of July sale. If they release L4D3 this year (or Q1 2011), I'll just wait again.