I bought a used one this year from a friend that was moving. It's great, but I agree with Marty, I don't use mine for exercising. Mine only goes in one direction, so I don't think it would be good for them to spend extended amounts of time on there. I do use it for letting horses dry after bathing, and to let them stretch their legs while I clean their stall. My B gelding, Fire, he figured out really quickly that he can make it stop by stopping, (too smart for his own good!) so if he is on there I have to watch him every second, lol!
What would probably be cheaper for you, and more effective for exercising, is building a roundpen,a nd work your horses in that. JMO