Newegg offers the Rosewill AC600 Dual Band 802.11ac Wireless USB Adapter, model no. RNX-AC600UB_v2.0, for $10.99 with free shipping. That's the lowest total price we could find by $2. It features up to 433Mb/s + 150Mb/s transfer rates.
Also available is the Rosewill N900 Dual Band 802.11n Wireless Adapter, model no. RNWD-N9003PCE, for $23.99 with free shipping. That's the lowest total price we could find by $9. It features three detachable antennas and up to 450Mb/s + 450Mb/s transfer rates.
Update: Specs corrected.