I'm just glad I managed to obtain/re-obtain some of the nintendo-published Gamecube titles before they show up in price.
Paper Mario 2 used to be, on average, $15-$20 used, now it's $25-$30 used.
Melee is probaly one of the biggest offenders, as it managed to nearly double in price over the last year.
And don't get me started on Pikmin 2. Yes, the Wii release made it so it doesn't usually exceed MSRP, but the price seems to flip-flop like crazy regardless. A few weeks ago I saw a few complete copies go for $25-$30, now it's back up to the $40ish it was around the time the Wii version came out.
In comparison, the average N64 cartridge is $15-$20 even for stuff like Mario 64 or Donkey Kong 64. Mario Kart 64, the Mario Parties, and Smash 64 are pretty much the most expensive non-Rareware Nintendo games, though Conker still seems to the most expensive. Those are just my Amazon observations, I'm sure the eBay situation is far worse.