For example if you are selling a 10 year old PPP ISH mare, then search for others listed as "for sale" in search tool and have a look at the prices going there. You're not going to want to put them up at the highest end if you want them to sell quickly.

Having a look through your barn also, your horses are mostly SSS or below, which means that they are not very good for showing or breeding. Your best bet for pricing these would be to put them for auction starting at 1k and see what sells.

Hope this helps :)