Cost of License

I am donating some old refurbished. PCs (C2D or even older) to the community (KiDs) somewhere in next week. Two KiDs are familiar with Linux and Libre/OpenOffice, I am very happy with that. Still, I am looking forward to buy at least 6 to 10 volume license either for Windows 7 or Windows 8, whatever is cheaper. The cheapest I could ever found is Windows 7 Home Premium -3 Users for $280.00 approximately from local reseller. I certainly can not afford. These KiDs are contributing and already getting a lot of used working HDDs either from ebay or kijiji for very... very cheap.

Any one has idea where can I get genuine license cheaper? Or I can give them without any OS installed, they are free to buy (certainly can not afford $280/3) and install whatever they want as a last option.