Walmart offers the Pyle 7" Mirror Monitor w/ Backup Camera for $42.30. Choose in-store pickup to drop it to $41.12. (Amazon has it for the same.) That's the lowest price we could find by $11. It includes a nightvision-capable rear view camera and clips on to your rear view mirror. The mirror features multiple video inputs for additional devices.

Update: The price has dropped to $41.12 before in-cart discount, $39.97 after. Amazon is now around a buck more.