The solo plumber also needs to put a lot of time in the office doing book work, scheduling jobs, restocking their van with parts, driving from job to job. If you are thinking about going into business for yourself either charge what your competition does - don't undercut anyone. If you can't figure out what your competition charges figure out what you need to live on and double it for a starting price and then get an accountant to look over your books in a few months. (though if you have no competition that is generally a sign that you have a bad business)