Walmart offers the Pyle License Plate Mount Backup Camera, model no. PLCM10, for $16.49. Opt for in-store pickup to avoid the $5.97 shipping fee. (Amazon charges the same with free Prime shipping.) That's tied with our mention from a week ago as the lowest price we've seen. (It's the best deal now by $4, outside of the mention below.) This camera helps you see behind your car and features a 92° view, RCA output, and 380x60 resolution.
A close price: Dealsbuy2015 via eBay charges $16.99 with free shipping.