We thought hard about this and in the end decided it's in the best interest of the nonprofits to host and manage their content for them. The $150 fee covers our expenses and alleviates the need for monthly payments.
Also it's worth mentioning that once a nonprofit has paid for something, albeit a very small fee, it makes them far more willing to put effort into giving us feedback.
Think of it as getting a free car but you still need to pay for the gas. Having a slogan of "$150 websites for nonprofits" dilutes the clarity of our mission and makes it look like we are profiting.