You should qualify for Microsoft Charity pricing from a Microsoft retailer like Consistent computer bargains, CDW Zones, or PCMall.
It isn't as good as the donation program, but still considerably less than full retail, and more flexible than the OEM product purchased with a server.
If that pricing is still out of reach, what are the things you want to do with your server? Is it possible one of the Linux based servers could provide you with the services you need on your network?
Dave