Whats a good inexpensive tree to put in my horse's pasture?

A nursery is a good idea. Stay with local trees and fast growing. What ever trees you choose make sure their is a fence around it, horses will chew on trees for the fun of it. It also keeps them away from the trees when they are taller, if you only have one or two trees in a large pasture they are lightning rods. I have seen groups of horse killed by one lightning strike. They will still get shade.