$13.76 on Amazon currently.
Knockoff may work, for a time. How long that time is.. well, only one way to find out.
Depends on what you are paying for 7093s, going with a different facepiece might make sense. The Moldex 7000 and North 5500 are both in the ~$15 range as well, and the P100 filters for either are generally easier to find and less expensive than the 7093s. If you're going the 3M route, I'd personally spend the extra $5 on the 6502 facepiece; it's a far superior unit both in terms of comfort and overall quality.