What is the average buying price of a horse in the US?

You should narrow the question down. Broke to ride? What discipline? Race horse? Broodmare or breeding stallion? Age? Breed? Show record? All of these things will greatly effect the price of the horse, not to mention what part of the states you are in. The price of horses varies depending on what area you're looking for a horse in. In the states you can find "free" horses (although they will probably cost you a lot in vet bills!), or horses at auctions for a few hundred dollars. You can also find horses for tens of thousands of dollars and the prices go up from there.