Newegg offers the refurbished Asus Dual-Band 802.11n Wireless N600 PCI-Express Adapter, model no. PCE-N53, for $21.99. This $20 mail-in rebate drops the price to $1.99. With free shipping, that's $19 less than you'd pay for a new one today (although most retailers charge $44 or more) and the lowest total price we've seen. It features data transfer rates of up to 300Mb/s, two external antennae, and support for WEP, WPA, and WPA2 encryption. Rebate expires July 31.
A 90-day warranty applies, although it's unclear who backs it.