With orders of $25 or more, Amazon offers the Design House Adjustable 35-55mm Door Viewer Peephole in Satin Nickel for $3.45 with free shipping. (Walmart charges the same with in-store pickup.) That's the lowest price we could find by $4, excluding the mention below. It has a 7/10" lens with 160-degree angle.

A close price: Home Depot charges $3.83 with free in-store pickup

Note: This is part of the Amazon "add-on" program and can only be purchased with orders of $25 or more.

Update: The price has dropped to $3.28. Walmart offers it for pennies more.