Hm... good question.
I'd think it depends on the area, market, discipline, etc. And I'd agree, a horse is only worth what someone is willing to pay for it so it depends on what they value.
I was told that every horse's base price is what it's worth by the pound, because that is the minimum that your horse is "worth". (minimum someone is willing to pay for them). Then on top of that i'd put training. A great unbroke horse is often worth less then a dead broke kid friendly horse. Then on top of that I'd put in performance/success in the show ring, but on a case by case basis. For an experienced rider looking for a successful show horse, performance will be more important then easiness and vice versa for a beginner. So at the top of the list are 1) push button super fancy A show winner and 2) grand prix level jumper. (my only experience is in the english world so that's where my examples come from!) A long list of achievements will add to a horse's price. (something with "Horse of the Year" on its resume will turn heads) Some people will value bloodlines/papers and of course the fad breed of the month, which can jack up a horse's price. And location has a big weigh in on all of that. Finding a green broke but fancy horse in the middle of no where will often be much less then a "hunter prospect" from a successful show barn. And a show horse from like, Arkansas will often be less then one from Florida. (not knocking Arkansas! But even an A show hunter from Arkansas is often worth less then one that spent the winter in Wellington!)