Haven't worked out pricing but aiming for the sub-$20 range.
I saw the MUXshield pop up a while ago but didn't even really associate it with the Centipede Shield, they do totally different things. The Centipede Shield is just good old true bidirectional I/O while the MUXshield is...a MUX, not surprisingly. The Centipede Shield is useless for analog and in fact steals two of the Arduino's analog inputs, while the MUXshield is insanely useful for expanding the possible analog inputs. However the MUXshield is not too hot with digital I/O, since you can't just tell an output to be a certain value and have it stay at that level. The Centipede Shield is fire and forget, no multiplexing or strobing. It's more like a row of shift registers, except you can set input pins and software pullups and do interrupts on a pin change, etc.
So you see the two shields are very different and will probably be used for different things, and therefore not "competition."
I'm not planning to supply the shield with headers installed, but the plan is to use 2x10 IDC headers and ribbon cable to make it very easy to break out all the connections to external devices.