ButterflyPhoto via Amazon offers the refurbished Linksys N600 Pro 802.11n Wireless Dual-Band Range Extender, model no. RE4000W, for $17.95 with free shipping. That's tied with our February mention as the best price we've seen. (It's a low for a refurb today by $7.) It simultaneously extends ranges for 2.4GHz and 5GHz bands. Features include up to 600Mbps data rates, two Ethernet ports, and WEP, WPA, WPA2, or WPS security.
A 90-day warranty applies, but it's unclear who backs it.