50 meters is a long distance. If you want to go with a USB-only solution, you can try using cheap Chinese usb 1.1 extension cables:
http://www.geeks.com/details.asp?InvtId=100E&cm_mmc=Froogle-_-Cables-_-USBCables-_-100E
they're 30 meters, only $19 each, and if you put a hub halfway between them you could probably keep them going.
I'm using one of these lengths to successfully control an Arduino solar heat sufficiency controller down in the house's boiler room.