Cafago offers this 4.3" TFT LCD Car Monitor with Rear-View Camera for $36.04. Coupon code "ZK3863MG" cuts that to $26.98. With free shipping, that's $24 off and the best price we could find. It features a 480x272 native resolution and includes a power cable and wireless video transmitter and receiver.
Note: This item ships from mainland China and may take two to six weeks to arrive.