Ignoring an early attempt with a 10 base hub, which only worked if the main machine was on, this has cost me a total of about 95 pounds:
Linksys 4 port wireless router £50.70 (Amazon)
Binatone Wifi WL1000 WIFI card £12.95 (Maplin electronics, special offer ending in about a week)
Linksys network card for laptop £15.00 (second hand)
Nic for one of the desktop PCs £10.00 approx - other has it built in.
3 cables - about 7 quid from the local computer market.
Getting it all to work was a bit of a pig - the automatic setup for the router gave me a few problems - but once that was sorted it was fine.
Later - forgot to say that adding a wireless access point to the original network would have cost about ten pounds less than the router, and would have been much slower, so this was a VERY cost-effective solution.