Newegg offers the refurbished ASUS Selectable Dual-Band 802.11ac / 802.11n Wireless Adapter, model no. USB-AC53, for $19.99 with free shipping. That's $10 below our July mention and the lowest total price we've seen. (It's also $28 under the best price we could find for a new adapter today.) It features WEP, WPA, and WPA2 encryption.

A 1-year warranty applies, but it's unclear who backs it.