I call a shareweb, based on the principles of shareware. By keeping costs to a minimum, I can solicit the most modest of donations. There are a couple reasons. I want to create a community of people who are united in their commitment to education for all people and who want to support a good cause for its own sake. Second, I don't want membership donations to be perceived as a money-making proposition.
All donations go directly to support and improve the site.
I already make plenty of money as a teacher to support myself. Third, if I charged more for memberships, I'd have to offer more. I really want to keep it simple. Finally, anyone is welcome to make additional donations in whatever amounts and currency they please. It all helps.