Anywhere from $50k to $150k, depending. Not an exorbitant amount of money, but on the high end of manual labor jobs and great for a guy who got mostly C's in high school. Not that you really need to be making that much money—how expensive are coffee and donuts anyway?
That'll be $37.50.
But do some math. If you work for a locally big plumbing company, they pay you a salary and you get paid whether there is demand for plumbing services or not. Think: a grand or so a week after a while, i.e. after you are skilled enough to be sent out on jobs on your own.
But if you have your own shop, things can get big quick. If it's just you working and let's say you can get enough demand for your services so that you can bill 30 hours a week at $65 an hour, that is almost 2 grand a week or a hundred grand a year. Like double of what you'd make working for someone—and you get all kinds of tax deductions to boot, i.e. your car, your phone, your wrench. (And to BILL 30 hours, by the way, it means you work like 50 because just driving to and from a job eats time.)
So then spin things forward and let's say you're The Man! (or The Wom!) and you now have 20 plumbers plumbing for you. Each plumber lets you bill $100k a year and each plumber costs you $70k a year if you include salary and training and benefits and stuff. And then you yourself plumb and add on another $100k yourself in billings, all of which you keep. The math: 20 plumbers giving you $30k a year in profits for $600k plus your own $100k. Nice gig. And VASTLY more than whatever any reasonably successful doctor or lawyer makes.