Matt, you're forgetting that add ons for other games arent free on PS3 either, so it's not just MS or Valve doing this to L4D. The add on prices for any game on any system are so the companies making the games can get some return on their hard work so if they used achievements as MS points, theyd never make money. All it would take is to go out and rent a shit kids game like "kung-fu panda" and horde the achievements. lol I mean i cant say much because I have over 20k achievements, but really not a way for game makers to make money. They already are having a tough time spinning a profit because of companies like Game Stop selling used copies of their games (which they dont make a cent off of) so whats a few extra bucks for an add on to a game you really enjoy?
And even though I have a PS3 now too which offers free online play, i have no problem paying $50 for MS Live. Go to best buy and get a 12 month card and you get an extra month free. So for 13 months it only comes out to 3.85 a month which isnt that bad.