Amazon offers the 3M Rugged Half Facepiece Reusable Respirator, model no. 6502QL, for $12.67. Add it to cart via Subscribe & Save to drop the price to $12.04. With free shipping, that's the lowest total price we could find by $10. It's medium size and features Cool Flow valve technology, which purports to help reduce heat and moisture.
Be sure to cancel Subscribe & Save after your order ships to avoid being charged for subsequent orders.
Note that it doesn't come equipped with filters.