If you have a market for koi, and go about it the right way (selectively breeding/culling to get top notch fish) then it seems promising. As Roger states, JCO seems to be doing this with success.

I keep thinking of adding a koi system some time in the future, not for profit, but purely for aesthetic/relaxation reasons (nice water feature that doubles as an input for plants). I started my AP efforts (as well as my other backyard growing ventures) solely with the goal of feeding my family quality food that is independent of the machine in place. Now, I have been tempering this with aesthetic considerations to feed the soul, and koi might just fit the bill.