P.S. One reason why I retain our dojo as a free dojo, rather than charging, is that progress in aikido can be very slow, especially at advanced levels. However the continued practise is worthwhile since it is body conditioning rather than learning. Also, long term students actually give alot back to the club, free of charge.
If you've ever tried to start a club from scratch you'll know that the level of training is heavily dependent on the capabilties of the students. I think this is not as pronounced in many other activities which are fixed and not dependent on uke/nage interaction (e.g. yoga).