This is a mouse that was once broken but is now healed. (A mechanical switch on the circuit board was broken; upon fixing that, the mouse works almost like-new.) I was actually quite surprised at the extreme simplicity of the actual mouse hardware - a couple of resistors, capacitors, an LED for the sensor, and the sensor chip itself. It seems to me that the costs for building such a mouse must be rather cheap indeed. In fact, I wonder now about the optical trackballs out on the market today - mine (the Kensington Expert Mouse) is to all appearances just an upside-down optical mouse with a ball and a couple of extra buttons; would it be perhaps possible to construct my own perfectly-functioning trackball with only cheap optical mouse parts?
Fascinating as that may seem, there are indeed more exciting projects to attend to first.