How much did horses cost back in the 1600s?

Most likely under $10. At this time in our history there was not much money 'floating' around. Like most things in the era most items acquired were bartered for. Basically an exchange between two or more people. Example:If a farmer needed a horse to pull his plow he could trade crops grown for a payment on a horse. The barter could even involve labor. The farmer needs a plow horse and his neighbor, who happens to have a young horse, needs the roof fixed on his barn.