I would prefer not pay a monthly fee, but could be an pay version with some extras which are for some people interesting.
You could also think to add some adds on the administration panel e.g google addsense.
so user / admins could click on it and you will get some money for it.
Or add there a donate-button there.
Ask some server providers to donate e.g 1und1, Hetzner, IP-X, serverloft, euserv etc. they could do some howto install for customers.
A gift shop would be nice e.g some cups T-shirts penguins, lamps, mouses etc.