Walmart offers the Gibraltar Elite Medium Mailbox in Black for $13.97. (With price varying by ZIP code, Home Depot charges the same in White via pickup; and Amazon charges the same with free shipping for Prime members.) That's the lowest price we could find by $6. It's constructed of rust-resistant galvanized steel.

Update: The price has increased at Walmart and Home Depot; however, Amazon still offers it for $12.39 with free shipping for Prime members.