I'm not sure if anybody is gonna beat me on this one ... $650 on one front-row ticket to see Radiohead in Cleveland on the Kid A tour. It was the first time I had ever seen them and I just went all out. I had to eat bread sandwiches for the next 3 months, but it was worth it.
Yeah, it is pretty ridiculous to spend that much money on one show, but I was like 18 years old and eBay has this way of just driving you INSANE over winning an auction (for example: playstation 3 systems are selling for over $5,000 right now ... go check it out). I was working part time as an IT guy at my dad's office ... making like $9.00 an hour, too, so that was pretty much an entire paycheck-and-a-half.
The show was AMAZING, though ... so yeah, I definitely think it was worth it. After all, what is money other than something we get and give to make our lives (hopefully) better.