Lochlan, this is a result of spineless store sellers letting BIN copies sit for sale for months on end at $99.99.
Everybody knows Air Zonk is not worth $100, but by doing so they get the less-educated folks who use eBay as some kind of "value guide" to buy into that mentality which in turn causes prices to go up all-around. It's artificial inflation brought on by scumbag store sellers, in particular GameSquad and one other who's name escapes me at the moment. The pioneer of this tactic back in the day was Bullseye, although the inflation he prompted was at the bottom of the laundry list of problems he had.
Like Necromancer said however, prices fluctuate. Someone here bought a CIB copy recently for like $40. Cheap copies, or rather, copies at a fairer price pop up from time to time. Just be patient.