Breeding, resume, temperment, and age are the 4 biggest factors to the value of a horse.
Breeding: Your horses breed and bloodlines. What kind of horses were your horses parents? Were they grand prix quality horses? Or was your horse the result of a 'backyard' bredding? Is your horse registered or registerable? Registration can count up to half the horses worth sometimes. I've seen PMU horses with no bloodlines jumping 4', selling for 10 grand, but a spruce meadows hanoverian jumping 3' selling for twice that. This includes movement and type, body structure and confimation.
Resume: What does your horse know, or has done? Is he a back yard pleasure horse, western, english, jumping, dressage, cattle, kids, other animals...etc. The more the horse has done (done WELL that is, any idiot can kick a horse over a log but it doesn't mean it has experience jumping) the more it's worth.
Temperment: the horses attitude. This can make or break a horse. You can have the fanciest warmblood in the world, but if it's very difficult and dangerous, you may never make it past training level dressage.
Age: If the horse is great in all these areas, but is over 15 years old, their worth can go down expodentially every year. Buyers want to be able to ride their horse for a long time, not just 2 or 3 years before the horse begins to lack ability, or needs to be retired. I've seen horses as old as 18 go for over 10 grand, but if the horse was 10 years younger with everything else the same, it would have been woth 30k.