There aren't a whole lot of choices in the low-cost fisheye world, but you do need to ask yourself the following three questions; you'd ask the first two contemplating any fisheye purchase, the third one is where the "cheap" bit comes in:
Do I want a circular or a diagonal fisheye?
Do I ever plan on shooting full frame?
Am I willing to put up with the inconvenience of a manual-only/adapted lens?
Circular or Diagonal
There are two main types of fisheye lens: circular and diagonal, and they deal with how big the image circle of the lens is vs. the sensor/film frame. A circular lens projects the lens's image circle completely within the frame (so you get a circular image on a black rectangle); while a diagonal lens projects a circle large enough to encompass the entire frame (i.e., you get a rectangular image with corner-to-corner coverage, hence "diagonal").
The angle of view of circular lenses is larger than that of diagonals, often encompassing 180° across the circle, but the image is much more unconventional. Diagonals typically do not have 180° coverage, and if you read the specs of diagonal fisheye lenses closely, you'll note most manufacturers will say 180° diagonal coverage--that is you only cover 180° from corner-to-corner, not across the frame.
Full-Frame vs. Crop
The question of frame coverage gets further confused by the issue of whether the lens is designed for crop or full frame.
A circular lens designed for crop (like Sigma's 4.5mm DC lens) becomes a tremendous waste of sensor real estate on a full-frame body. A circular lens for full frame (like Sigma's 8mm f/3.5 DG lens) becomes a not-quite diagonal on a crop-body camera--exhibiting dark corners. And a diagonal fisheye designed for crop (like the Samyang 8mm f/3.5, Nikkor 10.5mm f/2.8, or Sigma 10mm f/2.8 or Tokina 10-17) might mount on a full frame body, but will vignette and not cover the entire frame.
And then there are the oddball lenses like the Canon EF 8-15mm f/4L fisheye zoom, which is designed for full frame, but is circular at one end of the zoom range and diagonal at the other.
The (as Roger Cicala terms it) "Rokibowyang" 8mm f/3.5 fisheye lens is a common choice for those who want a lower-cost fisheye. It's certainly a great improvement optically over the previous budget choices, the cheap Russian-made Zenitar 16mm f/2.8 (diagonal, full frame), and Peleng 8mm f/3.5 (circular, full frame) lenses. But it is designed for a crop body, so it becomes less useful if you plan on moving to full-frame in the future, and possibly the Zenitar 16 or the Peleng 8 might be worth it, if you're willing to take the image quality hit.
The Samyang is actually a really good performer with the added benefit of mapping stereographically rather than equisolid (it's a little less fishy and more natural looking than your typical fisheye).
But all three of these lenses, the Samyang, Zenitar, and Peleng, are manual-only lenses. That's why they cost so little. There are no physical/electronic linkages to the camera, so they must be manually focused, the aperture must be manually set on the lens (aperture ring). You can't shoot in any modes other than M or A (because the body can't control the lens's aperture setting. And you won't have accurate metering (you need a D90 or higher end body to get accurate stop-down metering with a non-CPU lens). And the lens EXIF information will be missing. You might add an AF-confirm chip to the lens to get AF confirmation and some limited EXIF input, but it will not equate to the convenience of using a lens designed for electronic mount communication.
Footnote: the APS-C dSLR version of the Samyang 8mm fisheye is NOT the same design as the mirrorless NEX/Fuji X 8mm f/2.8 (or micro four-thirds 7.5mm f/3.5) fisheye. The mirrorless versions map equisolid and are considerably smaller.