I suppose you could eat a koi if you really wanted some awful fish. Koi are part of the carp family, (basically overgrown goldfish to put it in the simplest of terms.) so they would not be good to eat, though I have been told that the Filet o fish at McDs is basically a filet o carp, but I digress. But to answer the question, can you eat a koi, sure you can, but it won't probably taste all that great. Do people normally eat koi, not in my experience. They are more for looks and having in ponds.
BTW, apparently they can cost a little more than $300, some going for $1000, and more! (check out the site
http://www.coloradokoi.com ) I mean the fish was 15 years old, and these things can live 30-40 years...along those lines, who would eat an old cow? The meat would be tough and probably not as good.