A best bet is the Aller-Ease Bed Bug Pillow Protector in Queen (pictured) for $6.64
or King for $8.69. With $4.99 for shipping, that's the lowest total price we could find for either size by at least $6. This 100% polypropylene pillow protector purports to inhibit dust mites.
Aller-Ease Waterproof Pillow Protector - King is also available at these stores