Akodi via Amazon offers the Anewkodi AC600 Dual Band 802.11ac Wireless USB Adapter for $18.99. Coupon code "EJBDC5NL" cuts it to $10.99. Plus, Prime members bag free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $8.06.) That's a buck below our mention from two weeks ago and $1 under the lowest total price we could find elsewhere. It features a long-range 2dBi antenna and up to 600Mb/s WiFi speeds on 5GHz and 2.4GHz bands.
Update: Specs corrected.