I am just now seeing this thread. It does not seem fair that you need to spend so much on the upkeep of a site that benefits so many of us. I am an "afterschooler", but have extensively used resources and tips from WTM since I discovered it 3 years ago.
I think that it is reasonable to display ads for visitors who are not logged in - every time I google questions on a curriculum or try to compare 2 different curricula, WTM discussions pop up on my search. If you don't mind, I have a few suggestions for you to monetize WTM and it's forums:
1. Affiliate for selling curriculum of publishers directly from WTM website (I would personally buy all my books and pdfs through your links if that would help you). There could be special categories for the recommended curriculum and then others for other books.
2. Online classes from PHP (I have seen another thread about this being in the works)
3. Subscription to WTM boards - I suggest that you use a business model similar to the one used by the app company whatsapp - free for 1 year and from then on, a very nominal amount like $1/year. I believe everyone can afford that amount.
4. Donation button on your site - please do this at the earliest. A lot of us have already requested it
Thank you for WTM.