To be honest, I'm a little confused. I don't know anything about this 501 c3, but it sounds like its something for tax purposes to prevent from having to report income or something like this.
At any rate, what has me confused is that I'm not sure if you are trying to promote your work, of if you're trying to offer a not-for-profit printing business, kind of like an internet cafe where customers come in to use the equipment and pay for the time they use.
With large format printing, there is a learning curve, so its not like someone can just come in and use it. So this means someone, like you, has to do the prints for them... no? Its not like a woodworking shop as an example that people could rent out just so they have access to space and tools that they already know how to use.
So if you're looking to also staff the area with the printer, then this is a huge hit in terms of time. Charging $2.50 per square foot seems reasonable if you're not looking to make a profit given that outlets I've seen generally chart about $10 or $11 per square foot on the cheapest paper, but these would be the high end places. Mind you though, at 2.50, this would only be good for the cheapest paper. Since the ink is roughly $1 per mL which is about per square foot, when you add in your paper, you're not making much. So on second thought, for this price, you're only covering costs, but not really the depreciation of the printer, and certainly not any man hours.
Anyway, so without knowing exactly who will use this printer, how it will be used, and what involvement you will have, its difficult to see what a minimum price should actually be. But now that I think about it, this 2.50 price does seem quite small.