bloodlines, breed, training, age, health, bombproofness, potential, past performances, intelligence, willingness, level of showing, conformation, discipline specialized, etc
A horse's value really depends on his potential, his origins, his health and his past performances.
In auctions, young horses are valued mostly depending on their origins, and their apparent natural talent for the sport (rapidity, flexibility, intelligence, power…).
When they grow older, the price is mostly based on their past performances, their potential to become better, their health, the fact that they are easy to ride (very important factor, especially to sell horses to amateurs).
Other factors can make a horse's price be higher, for example if it was tried by a renown rider, if many people want to buy him, if it is a well known rider or merchant who sells him, if there are some commissions in the price...
Definitely age and skill/level of training, soundness as well. Although you can get a 16 year old horse with more go than a two year old, lol believe me, I went through that😂
Many people fixe a price depending on the horse's origins, which is pretty stupid because usually, it doesn't mean anything. You can get good horses on a reasonable price, but I recommend to look for a young one, if not the prices are really too high :)
First of all, I think about the type (a KWPN will be more expansive than a racecourse). Of course, that's a general fact, some of them will be cheaper or more expansive :) And the discipline and capacity of the horse :)