My Linux server tutorial course site http://GREP101.com brings in $700 on a good month and $100 on a bad one. Simply a reflection of the fact that I don't have the necessary things in place to drive consistant traffic to the site - when traffic comes then I get good conversion and a full course, but if not, not.

I keep it going mainly because the feedback is very encouraging, (people really do get a buzz out of learning this stuff!). It requires only a small bit of work each week to keep ticking over, but honestly it doesn't make enough to justify that if money was the only issue.