Amazon offers the Rokinon 8mm f/3.5 Aspherical Fisheye Lens for Canon (pictured), Nikon, Olympus, Pentax, Sony Alpha, or Sony E-Mount Digital SLR Cameras for $249 with free shipping. Outside of the price below, that's the lowest total price we could find by at least $31. It features an ultra-wide 180° diagonal field-of-view.

Of note, BuyDig offers the similar Bell + Howell 8mm f/3.5 Aspherical Fisheye Lens for Pentax DSLR Camera, model no. BH8M-P, for $249 with free shipping.