They have many phone cases on thingiverse, they are easy to print.

Depending on the material it could cost up to 50cents to print and 1 Hour of time.
With a little experience you can sell customized cases.

3D Printers with Aluminum Frame starting at $150.00 and going up should not take long to recoup you investment.

Remember though this is a starting with a Kit you have to assemble.
Mostly assembled start about $500.00 20 minutes to assemble for a beginner.