This seems an odd decision for non-profit and edu. I think you will lose a lot more in goodwill and new users than you will save by charging more. Extremely odd gamble you are taking.
Your best case scenario would be for these groups to down size land expenditures and continue using SL. But you just cost yourself, and the managers of these non profit spaces, a lot of credibility. I certainly don't blame anyone for looking to alternative grids.
LL, back to the drawing board. Please. Non-profit and Edu DO deserve a discount. Perhaps less than a 50% discount, but Something to encourage their adoption and continued use of SL.
I suspect you tax accountant can not find a way to write off your donations to these groups. Surely you can find a way to verify these groups as non-profit groups and thusly making all discounts deductible. Try starting here: http://www.irs.gov/charities/article/0,,id=96136,00.html Also, a trick I use in my business with non-profits. Bill them the Full amount, and then return them the Discount. =) This leaves you a tidy column of Discounts to write off.
Alisha