The most expensive Koi ever sold went for $2.2 million dollars. Owning a prized Koi is a symbol of status in Japan and champion Koi go for around $100,000.
This is the $2.2 million Koi named Alexandria. She was the 2008 All Japan Grand Champion, owned by Andrew Fillipowski from the USA according to a quick search. I have no idea how they would crown a champion of being a fish though.
I used to have a small pond with some 8" Goldfish and small Koi. Two separate times I walked out to find an 8" Goldfish eaten almost entirely most likely by raccoon.