A tenured professor though can get $20k per class or more, assuming that he/she is only teaching and not involved in any research (most start with 3 courses per year). Of course many tenured professors also bring money to the university through grants and whatnot, but that is not always the case. I know of some that get more than $250k a year just to provide prestige to the department and maybe teach a course now and then.

In any case, it doesn't matter if the class size is 10 or 50. You still need to prepare the same material, you still need to be there for the same amount of time. $3500/class sounds like a bargain to me. A TA earns more.

I buy the argument for community colleges. $3500/20 = $175 per student per class. The courses are cheap, so it's a wash. I disagree with this for private colleges where the cost per course can be $3,000 per student.

Many adjunct professors are people who have full-time jobs doing something else. A friend of mine did this one semester to make some extra money. He worked full-time as a structural engineer, but took a long lunch three days a week to teach Introduction to Statics at the university we both attended. He said it was fun to do for one semester, but he computed that he was only making about $10/hour when you included the time to prepare for lectures and grade papers.

In my experience, teaching has a way of ballooning to fill whatever time you have. Teaching one class feels like just as much work as teaching three, partly because either way you have to be available to students outside of class - office hours, answering email, and so on. I can't imagine teaching a single university class "for fun" while holding down a full-time job. Certainly it's not something I've seen very often.

Some people Adjunct to improve their resume too. NYU's school of continuing education has a lot of underpaid adjuncts who get residual benefits for the prestige of being NYU faculty. But nobody lives in Manhattan on NYU adjunct salary alone.

Adjuncts are especially cool for this reason in fields with a significant professional application.

Oh no! I didn't mean to imply a good living, just that those numbers make sense. But as has been pointed out, the school she was at was likely charging a lot more than $200/student.