[EricH]

Why build when you can buy? http://www.andymark.biz/ has gearboxes. Maybe you can get plans from them, or just the gears and make your own box.

Build a cheap gearbox--ouch. You'd need at least a manual mill. Then the material for the sideplates and axles. Then the gears and output sprocket. Assuming you have the mill, you'd be looking at at least a couple hundred, I think. That's about what an AM gearbox costs.