I managed to score a copy back in 2011 for about $15 and it ended up becoming my second favorite Rockstar game after Red Dead Redemption.
The only reason I traded it was because the copy I picked up initially didn't have the map, and I found one that did, so I traded up.
Other than that, this game stays in my collection.
I'm convinced I'm not alone, so that's probably why it's getting rather expensive.
Also, it seems that GameStop, at least, have hired someone who seems to know something about demand and rarity of certain games. But I dunno, they're still charging $35+ for a used copy of L4D2, so...