Shift 2 is an excellent game, though I preferred the first one more. For $15 I'd definitely purchase it even if I already had one of the titles (but that's me). All four games are easily worth $15 (or more) each in my opinion. (I paid full price for all four games - total cost £120 or $180).
Yes you are right, the game will be available again at that price. I've already had two copies for free via Steam for pre-ordering other games. Already given those away to other giveaway community members though.
The games industry must be really suffering at the moment with games that have only been released for several weeks selling at half price or more. Prior to the recession (pre 2008) game prices held for much longer. Arcade prices were around $20 each, wheras now around $7 or less is the norm and AAA games rapidly reduce in price over much shorter periods and are often found at lower prices than their arcade counterparts; (apart from the more popular games like the Call of Duty series). The main game seller in the UK Game UK recently closed all of their sales outlets in the UK. you can only purchase games online now. I used to enjoy perusing the game shelves for new PC games. Though over the last several years the shelves have rapidly shrunk w.r.t. PC games, with Nintendo, X-Box and PS3 games taking over most of the space that PC games used to occupy. The last time I visited a Games shop several months ago, the space they gave over to PC games had shrunk from 10+ shelve sections to less than one section. It makes me wonder how long PC gaming is going to survive, especially now as the new platforms such as the ipad are taking the world by storm and both the X-Box and Play Stations will be upgrading to new chips in the not too distant future bringing them into line with present PC graphic standards.
(Scuze the waffle, I'm passionate about PC gaming, so can't help myself sometimes) :)