I tend to agree, as that turns out to 25$ per hour for 10 hours class + 10 hours prep.
6 meetings*3 hours/meeting=18 hours of just class time.
When I prep workshop-style classes from scratch, I usually require as much prep time as class time so that would be another 18 hours.
36 hours is a full teaching week for me, so I would charge around a thousand dollars (good round number close to $30 an hour). If I were going to do most of the prep anyway for my own purposes, anticipate enjoying the experience, and feel charitable toward this organization, then I would cut a deal of maybe $900 or even $800. If I were going to do a lot of special prep for this class--prep that probably will only be useful if I ever teach this particular class again-- or something twinged as being unpleasant, then I would charge $1200-$1500.
My general rule is that if I'm not doing something for a token honorarium because I believe in the cause, then I will be paid an amount reflecting my effort. Otherwise, sure, pay my expenses and we'll call a hundred bucks good.