I paid 17.95 for it too... however I'd pay 20 for it no problem. Maybe even 25. And I am broke.
Why?
Indeed, it has last year's graphics (which, by the way, are still pretty spectacular at 1680x1050 with HDR and AA's) and sure it's just a short episode... But Valve have proven to be second to none with gameplay. Half-life and Half-Life 2 took gameplay to a whole new level with it's immersive story and awe-inspiring level design. Compared to it's contemporaries such as FEAR (incredibly repetitive level design, uninspired story) or Doom 3 (can they think of anything else other than demons popping out of closets or warping in?), Half-Life 2 was much more intense, immersive, and well, downright fun. I'd pay the same amount for an Episode from a top-rate developer such as Valve rather than a full game from a someone who goes for pretty graphics over the requisite elements of gameplay (story, pacing, level design, realism, etc.).