That is a complicated question with no black and white answer. :) Basically, a horse is worth what a person will pay for it. It's kind of like buying a car. Someone told me once that the bottom dollar a horse is worth is what it would sell for by the pound, which is like buying a base model car. The more "fancy gadgets" it has, the more it's worth. So let's just say you have a nicely broke horse. Generally worth more then something that's not. Well most people don't want a super old horse or a completely inexperienced one. So a good age and experience can make it worth more. Not every horse is trained in a specific discipline so those who are can be worth more. Trail horses tend to be cheaper then say, a well schooled dressage horse. What brings in the big bucks is how competitive they are at shows. A horse that can jump very high, a horse that moves very well, a horse that gallops really fast, etc. They're worth even more if they are very fancy AND easy enough for a beginner to ride and win. So, someone who is looking for a 3"6 horse to show and win on the A circuit, win all of their hack classes, and easy for a not very talented rider is usually VERY expensive! But people are willing to let certain things go. For example, a professional grand prix rider is looking more for ability to jump well more so then easiness to ride. I train mainly kids ponies so I look for a prospect that is laid back with a good brain, then I look for quality of movement. I'd MUCH rather have a less fancy horse that will take care of a kid then a super fancy horse that bucks kids off.
Does all of that make sense?