It’s not that simple things don’t exist for cheap. It’s that the knowledge to use them to do something non-standard is not cheap. Assuming no one has built something that does exactly what you want, you need to take two or more things that weren’t specifically designed to work together and make them work together. Since you want it to work from a computer, expect to be writing code.
You need an arduino (or similar), a few brushed motors, some relays, some wires and cables, a power supply, and the mechanical parts to put the whole thing together. If you know where to look, you can easily get that all for much less than $300 (assuming we’re talking about fairly small motors). But getting it to work together is non-trivial. You’ll have to be able to write the code and wire up the motors.
I’ll add that I don’t think I have a good idea of what you are trying to do, as you haven’t explained it too well. What I got was you want to be able to control a motor with a computer.