In this tutorial I will introduce various motor controller circuits for DC motors and stepper motors (uni-polar and bi-polar). Where appropriate I will give an idea of guideline costs - based on prices at Digikey as of summer 2008. These costs will assume that you are adding the controllers to an existing robot project and so will exclude the costs of the motors, wiring, soldering equipment, circuit board etc and focus purely on the cost and capability of the crcuit.
Each controller has an Eagle schematic but the implementation ie strip board, matrix board, hand crafted PCB is up to you. For those unfamiliar with Eagle then I suggest that you download the free/light version.
Familiarity with the concept of 'H-Bridges' is assumed because it is explained very well by others, either on this forum or via Google or Wikipedia. In essence: it is a circuit that can push current through a motor coil in both directions (ie it can make a DC motor go 'forwards' or 'backwards'). Equally concepts such as Pulse Width Modulation (PWM) are assumed or see my tutorial