Cheapest would be a wired router like this
http://www.amazon.co.uk/exec/obidos/...095832-2701266
along with 5 differing lengths of cat-5 cable, available most places.
Best = Wireless
http://www.amazon.co.uk/exec/obidos/...095832-2701266 but is more expensive as you need to buy adaptors for the pc's to receive signal
Wired= About £60 - but loads of wires everywhere
Wireless= About £180 in all - can be less reliable but means no wires or anything.
Also I'd suggest a higher connection between 5 pc's.