WTF, slashdot ate the top paragraph, sorry, let me try that again.
If a program can be sufficiently complex so as to create the program for the 3d printer to print the key with less than 5 dollars of effort, they could just as easily create tool paths for a CNC mill.
If you have to create the 3d printer program, well that's not any easier than creating the CAD drawing for the CNC machine. I can build a CNC mini-mill for 1500 dollars, so they are cost competitive (in relation to this topic) with the 3d printers also. Side bonus: steel key is much more resilient than plastic.