Army Guyz via Amazon offers the Soviet Russian Civilian Gas Mask for $4.25 plus $5.37 for shipping. That's $4 under our February mention and the lowest total price we could find by $13, outside the price below. It includes an NBC filter, cloth bag, and is available in sizes S, L, and XL.

A close price (with more stock): Trove of Goods has it for $10.49 $9.99 with free shipping via Prime. (Non-members can spend $25 or more to qualify for free shipping; otherwise, it adds $5.58.) It's available in sizes S to XL.