$80 for Mario Kart 64. The stores were all sold out except for one local used/import store who was scalping the game, but I had a friend spending the night, and it's what we wanted to play more than anything else, so I bit. It was totally worth it.
I suppose I also spent something like $130 upgrading the ram on my 486 from 4mb to 8mb so that I could play King's Quest VII. It stated that it only needed 4mb, but the original release was windows only, and 3.1 ate enough of the ram up that I couldn't get the game to launch. If I had returned it, I could have just bought the second release that had a DOS version. Sadly, the computer got fried when a friend tried to upgrade the processor for me as a christmas present, so the extra ram wasn't good for much besides the one game, and as much as I liked the King's Quest games, VII wasn't worth the expense. VI, on the other hand, would have been.