Amazon offers the 3M 8293 P100 Disposable Particulate Cup Respirator for $8.68 with free shipping for Prime members. (Walmart charges the same via pickup.) That's tied with our mention from a month ago and the lowest price we could find now by $3. It received the highest rating (P100) for filtration efficiency in a filtering face-piece respirator from the National Institute for Occupational Safety and Health. Features include a Cool Flow exhalation valve, foam face seal, and adjustable noseclip and straps.

Update: The price now drops to $8.40 in-cart at Amazon, or when you choose in-store pickup at Walmart.