It doesn't seem to stay in stock for very long. The $13 price point is part of Gamefly's used game sale, but I think the price only applies to the Xbox 360 version.
Bah! Just bought this game last week from Newegg for $25. Anyway, it's definitely worth $20 (and $25 imho) so snatch it up! Or get it $13 used at Gamefly, either way.
$10 is the sweet spot for "if I don't like it, it was only $xx", so I'm definitely in. Played the demo a while back and it was pretty interesting, but at the time I was more interested in Resident Evil 5.
Thanks!