There's actually a couple of sites out there that work this way. One I've looked at (emachineshop.com) allows you to download a program similar to AutoCAD, and after familiarization with the program, you can create your own dimensions in 3-D, choose a material to have it made from, like aluminum or plastic (not all materials work with all designs), and they give you an estimate of how much it'll cost, and you can order it. Here are two other threads that sort of dealt with this topic:
does this sound familiar to anyone? Digital fabricators and the toy industry
