Except you'd spend more on electricity than you'd have spent to just buy it.
Thats not including source material.
Thats not including the fact that the part you printed is going to be so weak, relative to the mass produced part that it doesn't stand a chance of being useful.
You only think its a good idea because you have absolutely no real world experience with 3d printing.
3D printing at home is a joke for anything other than AT BEST making lost-PLA (i.e. lost wax but with PLA) style molds that you can fill with a metal, and even thats HIGHLY specialized.
Note: I have both a 3d printer and a CNC machine, and 9 times out of 10, I CNC it rather than print it and do a pour. Nothing printed at home has enough strength to be useful. If you think even a 3d printed phone case would be useful, you really don't have any idea. One won't even last you through the work day before its coming apart.