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, the Samyang 12mm f/2.8 fisheye, which is designed for full frame, might be a better choice, despite being less fishy on crop and more expensive.
The Samyang 8mm for ASP-C 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).
There are the cheap Russian Peleng (circular) and Zenitar (diagonal) lenses, but the optical quality isn't as good as the Samyangs, and the cost savings isn't particularly large.
But these lenses 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.