Sculptor's Cut was a Blockbuster Rental Only deal. That's why it's so high.
I mean, look at how high Sonic Adventure: Limited Edition goes for. It is literally a limited version of Sonic Adventure, it's the same game with all the online functionality cut out of it, yet it goes for hundreds while the full version goes for peanuts.
Sculptor's Cut is at least an enhanced version of the original, with more characters and other stuff. I can actually see why someone would want it in terms of actually playing it. Things like SA:Limited remind me of when I see people get extremely excited by a Red Label NES game that they already have a Gray Label version of. I'm all about collecting stuff, I'm in the process of getting every Dreamcast game, but I just don't see the point in having two Sonic Adventures.