Yeah, the cost of all the mechanical parts also makes buying a controller, even one that needs repair, a bit easier.
My plan is for a brushed, variable speed pwm, reversable motor controller, that reads the signal from two ports of a normal futaba or traxxas type of transmitter, which is also pwm, and controlls two motors, for skid steering. But I won't go into too much detail about that cause it already has its own thread. As for the cost issue which is relavent here; I'll be using a L298 h-bridge chip (about $3) and a 18F1320 ($5ish), besides that all that is needed is a few resistors and small caps, a dip switch, a trimmer pot and a power regulator (total about $4) and a small perf board ($2), even rounding up to include solder and stuff thats under $20 for a customizable device, and I use the trimmer pot to make it so you can adjust the relative speeds of the tracks to account for mechanical variables and make it drive straight when you push stright up on both sticks. I've seen bi-directional controllers for as low as $8, but I would need two of them and I dont think thos cheapos are very adjustable or handle 4A. But anyway people get into electronics when customizing your own thing is worth more to them than saving time and money. And to be fair, I am using some economy of scale because im buying a premade motor controller chip to take care of 90% of what Im doing. I'm not just arguing to try to be right though, if you know of some speed controllers that could do this for cheaper, well it would deflate my project, but it would be good to know about.