Fortunately controllers are getting cheaper, at least in the US market. Ordered straight from china on Ebay is often the cheapest. A guy by the ebay name ecrazyman can be trusted to have good stuff.
If you are interested in larger stuff, like converting a motorcycle, it does get expensive though. While some early golf carts used brushed motors and switches to avoid having a controller, it worked poorly, and was very hard on batteries. So now all golf carts use controllers, even if using brushed motors.