for my ranch store I usually follow these rules
less than 3/6 genes= livery
3/6 genes = 5k
4/6 genes= 10k
5/6 genes= 15k
6/6 genes = auctions

although i mainly catch for the exp, so my goal is to sell as many wilds as fast as I can catch them so I can catch more. if you're dealing with horses with exp, I can't help you there, but I guess you could always put them in auction with half what you think they're worth as the minimum bid and see where people take it from there