How much would this koi fish tattoo cost?

you will most likely get a different quote depending on the tattoo artist. For three small words and roman numerals on my ribs I was quoted 120 (all black ink). So for something that large, detailed and colorful you are looking at a few hundred bucks. Closer to 1,000. I cannot tell you for sure. Go into a few shops and ask :)

and always remember! You get what you pay for.