The quality of koi (and therefore the price) is primarily determined by symmetry, shape, and pattern/color. Size really isn't as big of a factor.There are more big "backyard pond" grade koi out there than people who want them, so these koi are essentially worthless money-wise unless you come across someone who thinks a big koi must be worth a lot regardless of the traits listed, and will give you some money (which is possible on craigslist). I wouldn't charge more than about $10 each, but you can always start high and haggle with anyone interested.