It's $15 at Gamestop. They ran a B1G1 deal on Xbox games a month or two ago, and I think a B2G1 on all used games, so just wait until they have another if $15 is too much for you.
Find this game will be a hunt. Luckily, I found a copy at GameStop, just a few minutes after someone traded it in, in early-January 2008 for fifteen dollars.
I picked up a copy a couple years ago at Game Crazy for $15 used. Which it's still that price for the Xbox version there, while both GC & GS seem to want to charge more for the PS2 version.