Dollar1 offers the Ray-Ban Unisex Wayfarer Polarized Sunglasses in Black for $80 with free shipping. That's the lowest price we could find by $4; beyond that, many stores charge over $100. They feature 50mm lenses.

Update: The price has since dropped to $70 via coupon code "NEWADDICTION". With free shipping, the lowest price we could find by $14.