The most I ever spent on one game was $500 for an Asteroids Deluxe machine. ($520 if you count all the new bulbs and a ballast.) I don't remember where the money came from; I just had a chunk o' change burning a hole in my pocket and happened to find an arcade cabinet in a junk shop.
On a more Famicom-related note, my second place game purchase goes to a CIB copy of Summer Carnival '92 I picked up back in '08 for $475. According to eBay, I haven't seen much of a return on that investment, which is kind of surprising.
I will never spend anywhere near that much on a cartridge ever again. I just couldn't bring myself to do that these days considering how common counterfeits have become, and how good they can be. There was a time when you could buy a copy of Surprise at Dinosaur Peak on eBay and be fairly certain it was real, but that time is long over. The whole retro game market has gotten way too crazy for me. I'm going to stick to $10 Famicom cassettes from now on.