It really varies and also depends on how good you are and how fast you are. That and also how honest you are as well.
When I was an auto tech working for others it was like a plumber. I got paid a certain amount based on the work done. So if we were busy I made good money. If slow I made little to nothing.
Now when I worked for myself I did well. I did not do major work, engine overhauls, at the time as I was full time in college so mostly light to medium duty repairs and diagnostic. I grew up when most of the long term good techs were great at Carb work but not so at F/I. So I got called to come to some others shops and do diagnostic work for them on newer F/I cars. Also when I got busy I passed out the work to Techs I knew and could trust. I would diagnose the job needed, buy the parts, and then have the other person do the work. I had to do that when I got busy at school (finals, team project, etc?) or when I was so busy I could not do it all but did not want to turn down repeat customers that trusted me.
But I did not want to do that the rest of my life. I hated working for other shops as most did not meet my ethic standards and also got paid little for what was done a lot of time. When I had my shop it was good but I also had good weeks and awful weeks. That and my back would be sore; coming home with antifreeze in my hair, etc? was not fun.
There is nothing wrong with blue collar work, but you get paid as hard as you work. I knew a lot of older guys who had back issues amount other problems.