Mwave offers the Powerlink 802.11n Wireless Adapter, model no. PL-U2NA, for $7.99 with free shipping. That's the lowest total price we could find by $2. It features a 2dBi antenna and transfer rates up to 150Mbps.
mwave offers the PowerLink 150Mbps 802.11n Wireless USB 2.0 Adapter, model no. PL-14N, for $19.99 with free shipping. (PremierTek via eBay sells it at the same price.) That's the lowest total price we could find by $9. It features a max speed of 150Mbps, dual 7dBi dipole antennae, operating distance of up to one mile, purported 6x signal strength, and support for WEP, WPA2, and WPA-PSK encryption.