I'm looking to develop an affordable, totally open hardware, expandable quadruped walker robot over this summer. I have found that this sort of chassis is rarely found commercially available, I only know of about two examples, both of which are somewhat costly.
I'd rather see something at a sub-$100.00 USD price-point; I think it could be done, too - if you used the small 9-g servos, along with PCB material for the leg pieces. In fact, if you took this route, the entire "kit" could consist of a bunch of "break-apart" pieces on a single "board" shipped flat, with a kit of component parts (electronics and other hardware) to solder on the "body"; if you set it up right, you might even be able to have traces along the leg pieces to small right-angle mounted push-buttons at the "feet" end, to sense "pressure" and relay it back (or something similar).I'm not sure if $100.00 would be the manufactured cost, or after markup, but even if it were the manufactured cost, you could bump the SRP to $150.00 and it would still be a worthwhile kit (as long as all parts - on board stand-alone arduino controller, servos, etc - were all included).
I suggest you make a working quadrapod prototype. It could be something simple first made with small servos, craft sticks, and hot glue. Also study other's projects like below.