TRENDnet 300Mbps Wireless 802.11n Easy-N-Upgrader for $29.99 shipped.  Here is a screenshot for your reference.  Note that this is not a router itself but is more like an access point.  It gives the advantage of not having to reconfigure the network while adding wireless 802.11n (300Mbps) to an existing network.  Most places sell wireless upgrader for about $38.