As part of my $500 robot project, as I've posted here before, I'm using a mouse as part of the motor control interface. When you think of it, a mouse is almost perfect for this application. It containes all the hardware to interface the encoders to the computers and it follows a fairly well documented protocol.
Well, I was thinking, and what do you think of this: Using a USB\PS/2 mouse controller with scroll and three button inputs as a three axis generic encoder interface board. Depending on the software options, you could use the three buttons as zero marks for full X/Y/Z motion. You could use the PS/2 port or USB port. The whole thing could be built for a unit cost of about $3.00 (in moderate quantities).
Anyone think there is a viable market? If you're curious google for an "HT82M98A" controller. About a buck each in a quantity of a thousand.