Talking about doubling, tripling, (or more) in value is absolutely nonsensical in the horse world.
What you pay for the horse has almost no relation to what you sell a horse for.
What you can sell a horse for is directly tied to how much risk you took in buying a horse and in direct proportion to that, how much you’re able to prove what the horse can handle. In other words, someone who goes to the big shows and puts the miles on the horse will always be able to sell a horse for more than someone who only schools at home. And someone who demonstrates what the horse can do (i.e. horse can jump big jumps followed by video of the horse jumping big jumps at home) can sell a horse for more than someone who says a horse “can” do it with no proof. And so on.
But the ability to buy a horse for $5k and sell it for $500k, has no relation to that initial investment price. Which is why someone with a good eye can buy a horse for $500 and then put it on the market for $25k a few weeks later. No, the horse didn’t “increase in value,” but the person with experience and a good eye found it and put it where it could have been all along.
Realistically, every zero that gets added on is just one more degree of guarantee that the horse will do it’s job. Breeding is one degree. Training is another. Conformation and looks are another. A winning show record is yet another. Add up enough of those points and any horse moves into the higher range. Again, often totally independent of the purchase price.