Koi fish breeding can boost your pond’s population and extend the lifeline of your koi. Breeding takes many steps and can be costly, but if you’d like to grow more koi for your pond, or maybe even sell them, koi breeding may be for you. Once they reach a foot long they can be worth about $100 each. With your koi producing so many eggs, the amount of money you wish to make just depends on how much space you have to raise the koi.