Depends what cheap is.
I guess you could always use sprockets and chains, and they're free/near enough free if you know what you're doing and can find what you're looking for easily from bikes or whatever, but it all depends on your motor. What RPM is it? If it's a higher RPM, naturally you'll more sprockets and chain or gears to gear it down more to a usable speed, which will be more expensive to do if you can't find the right parts as scrap or otherwise!