Looks like just a Wifi module by itself sells for about $50:
Obviously Wifi could be cheaper, since you can get a USB WiFi adaptor under $20:
http://www.amazon.com/TRENDnet-54Mbps-Wireless-Adapter-TEW-424UB/dp/B000636JD8/ref=pd_cp_e_3
but that may depend on PC host processing for doing the protocol... The trick is how to hack that into an Arduino compatible system. If you figure it out, let us know!