Not all kois are worth money !! It depends on kind , color pattern , shape, defined lines and color separation, and location of color marks , you can get 15-20” kois for 50 bucks !! And some 15-20” kois will cost your 5000$ !! Do some research and find out what makes a koi worth money and what kois are garbage pond kois , sorry didn’t meant to sound so blunt , buts it’s the truth , I’m pretty sure of the old owner had some really high end kois , he woulda probably sold them before he sold the house , I did kois before , and Looking at your picture , honestly they’re not really high end , as for what I can see , just pond kois for nice fish scenery, sorry just don’t wanna you to read all these posts and get your hopes up think your kois are worth thousands of dollars