There are several factors that contribute to the 14mm lens being close to three times the cost of the 10.5mm
First, it's a full frame coverage lens, vs. a DX lens.
It's also a Rectilinear lens rather than a fisheye.
It has an aperture ring which adds to the build cost.
The G lenses seem to range in build quality. While I'm sure that the 10.5mm has good build quality, the 14mm uses more ED glass, and less plastic material. Look at the difference in size and weight, the 14mm weights a hefty 1.5lbs compared to the 11 oz 10.5, so twice the weight = more metal, and more glass.
I've used the 14mm but not the 10.5mm, and neither UW. The fisheye effect is sometimes interesting, but in general is not a practical day to day lens. One can use software to create a rectilinear image from the 10.5mm lens, but the end result is a crop.
There are some websites that review the 10.5mm and show the crop factor if you are interested in seeing the end results. The 14mm is a 21mm (35eq) on a DX body, and the 10.5 is about a 16mm fisheye, not really all that similar, but more so if you use the software to create the rectilinear image.
The 14mm was designed to replace the very popular 20mm f2.8 on DX cameras. The 10.5mm was designed to replace the Nikon 16mm fisheye.