What would be a good fish eye lens for my nikon D3100?

I would like it as cheap as possible

I'd look at the 10.5mm 2.8. Its designed for the DX format and if you use Capture NX2 it will automatically fix the distortion. I have used this lens and it appeared sharp from edge to edge on my D300s.

Low cost means low quality… A Nikon lens will cost more than the body did.
You can look on a site like amazon for ideas.

Http://www.sigmaphoto.com/shop/15mm-f28-ex-dg-diagonal-fisheye-sigma
If you can afford it. It's the cheapest Sigma fisheye.