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. However, the Samyang 8mm 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). The CA control, flare, and corner sharpness are very good for a fisheye. The sample images I've seen from it, at least to me, compare more than favorably with my Sigma 8mm f/3.5 circular fisheye, although possibly not as sharp as the Nikkor 10.5mm/2.8 or the Sigma 10mm/2.8 (otoh, both of those fisheye lenses are in the $600 price range).
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.
*Foonote: Other names for the Samyang 8mm fisheye include: Rokinon, Bower, Phoenix, Pro-Optic, Opteka, Walimex, Vivitar, Falcon, Bell and Howell, and Polar. For some odd reason, the Opteka is listed as 6.5mm, and the Vivitar as 7mm, but they're all the same lens. **
**Footnote 2: the dSLR version of the Samyang 8mm fisheye, however, is not the same design as the mirrorless NEX/Fuji X 8mm f/2.8 or micro four-thirds 7.5mm f/3.5 fisheyes. The mirrorless versions map equisolid and are considerably smaller.