Here you go. How bout $1.46. This assumes of course that your project has USB and you know how to handle the Ralink chipset. If not, just buy the TP-Link 703 and hook it to your arduino.

http://www.amazon.com/SANOXY-USB2-0-Wireless-802-11-Adapter/...

If you want the whole computer instead of just wifi, try this:

http://www.volumerates.com/product/tp-link-tl-wr703n-openwrt...

OpenWRT preinstalled for your convenience.