Ultra wide lens under $500.

I cannot find a ultra wide lens (prime or zoom) for a reasonable price. I don't want to pay $600 (actually $700 with shipping & tax) for Oly 9-18mm or $900 Panny 7-14mm. I tried to de-fish some Samyang 7.5mm  images I downloaded from Photozone using Fisheye Hemi but final result still had the fish eye effect.