Linksys offers its refurbished Linksys AC1200 802.11ac Dual-Band Wireless Range Extender for $18.99 with free shipping. That's $41 under the lowest price we could find for a new one. It's compatible with all WiFi routers and features dual 2.4GHz and 5GHz bands, a range of up to 7,500 sq. ft., Gigabit Ethernet port, and 128-bit AES link encryption.
Note: A 90-day Linksys warranty applies.