"Fun" anecdote on this: long ago, I used to teach Ruby and Rails, professionally. One of my old professors emailed me asking if I'd like to come back and teach as an adjunct for a single course the next semester.

The pay for the entire course was the same as our day rate was in industry.

Of course, as an employee, I wasn't making that day rate personally, but that's how far out of whack it was overall. It's really brutal. I would love to teach, but it doesn't make any economic sense: ironically, part of the reason I need the extra money is that I'm still paying off my student loans!

The private teaching industry does seem to be able to charge a lot of money, e.g. the coding schools and the (human) language schools. It's the research-oriented/public schools that don't have much extra money for teaching or simply don't care.