If you want to find out, how much your horse "is worth" for selling or for brood, there is an easy way to find out:
Go to the "Search" and enter all the options as if you want to search for your own horse.
age / gender / breed / maybe color / ratings
then click either
"for sale" or "for breeding"
and most important:
click the "lowest to highest" checkbox!
Then all compareable horses will show up. If no one shows up, lower your options and unclick e.g. color or age until you will find some horses.
Then you can go from that prices - either you set your price cheaper to make sure the horse will sell or get breed fast.
Or you want to mke some money and take a higher price and maybe lower it within a few days if it does not sell / get used.