Least expensive circular fisheye lens under 8mm?

I will be buying a Nikon D3100 soon and I really like the effect of fisheye lenses, particularly circular ones, such as the Sigma 8mm lens, but I can't afford them.

Does anyone know of any fisheye lenses that are 8mm or under and is sold for under $100?
They don't have to be high quality ones; I would just like fisheye lens for its effect.

And they should be able to be used with a Nikon D3100.

No and if they are they would be of horrible llow quality. I bought a bower for my d60 for $350 and it is a manual lens and works great.