There is an educational price for the OSX upgrade at around $70 which a library might qualify for. For multiple machines I think there are maintenance agreements that will keep your machines up-to-date for years (check Apple for the pricing which I think is based on the number of users: I'm guessin a Library has only one user account for the public).
As for the server OS, if you are using an Apple rack mounted server, I heard the OS X Server upgrade was insanely cheap ($10 or so). I imagine that there is/was some maintenance policy in effect for this software also if you are running it on some other computer.
For the record, I'm not enthusiastic about the pricing. If we are comparing Apple to Microsoft using something like Red Hat as the baseline, we see that in pricing of hardware+OS and upgrade pricing, Apple is closer to the latter than the former. As for the donation of Pentium boxes... well Microsoft makes more in profit that Apple makes, and the connection between Bill and Melinda Gates and Microsoft is no secret: sounds more like a charity being co-opted to further a business end of increased market penetration, than it does out of any good will on the foundation's part. Sad.