I do that same thing. Earning around $100 an hour, and I can do as many hours as I want, whenever I want to. I do this on top of my full time job, so I usually do a few hours at night or a weekend day here and there. That's similarly in the $60-70k extra per year.

What I do is embedded software for medical devices though, so beyond building there's a lot of documenting, reading documentation adhering to standards.