It's difficult to obtain one, with prices exceeding 700 euro...
Unfortunately this is true. In it's 6 year production run about 8000 of these lenses were manufactured. Unfortunately a number of these were lost due to damaged front glass and other defects. I have even seen a few copies with perfect front glass but damaged internal lens elements too and I have seen a few copies with broken hood petals which with this lens are part of the lens barrel.
I bought mine for EUR 200 last year, so if you search long enough you can find them. I know someone who bought one for US$235 earlier this year from ebay. So with a little patience it is possible to find one. I got lucky and was informed by a friend that the copy I eventually bought was on ebay. A few people I know searched for months or little over a year to find a good copy of the lens and as mentioned before, quite a few people I know bought damaged copies that were beyond simple repair.
Whether it is worth it to search for this particular lens rather then the later 16mm f2,8 fish eye? In my opinion it is. This version seems to be a little sharper across the frame compared to the 16mm f2,8 fish eye Nikkor. It is a lot sharper then the Zenitar 16mm f2,8 which can be bought for EUR 150 new. At least one person I know prefers the 16mm f3,5 Ai Nikkor fish eye which has improved internal baffling reducing ghosting and flare a bit. This person owns 3 out of 1500 copies made of the Ai version.
I own a pre-Ai version and cut the aperture ring to allow metering and use on my digital cameras. I never had problems with flare when the sun was in the frame so I am not sure how much of a benefit the changes made for the Ai version would have for my particular use.
Below are some examples made with this lens. The Fisheye Nikkor Auto 1:3,5 f=16mm
DSC_7274 by
b j, on Flickr
(btw, a nice feature for black and white photography is that the lens has internal yellow, red, orange, green and N filters where N is a clear glass filter. The filter is part fo the optical system so a filter must be in place to use it. Many of the 16mm f2,8 fisheye Nikkors are sold on ebay unfortunately come without the originally included filters because they were screw-on accessories that come in front of the rear element)
At f5,6
DSC_6517 by
b j, on Flickr
At f5,6 and near minimum focus distance (the glare/glow you can see in this image was added in post because it better resembled my state of mind at the time I took the photograph. The original image was too sharp for my liking, at least for this particular scene)
DSC_6456-Edit by
b j, on Flickr
At f3,5
Cherryblossom 1 by
b j, on Flickr
Cropped some
DSC_5028 by
b j, on Flickr
Sun in the frame
DSC_4757 by
b j, on Flickr
Shooting stars with the D800 at f5,6
DSC_9499 by
b j, on Flickr
Wrong EXIF here, but at f8 or smaller this lens can make very nice sunstars
DSC_2861 by
b j, on Flickr
(you can download all of these in full res by following the Flickr link, as well as find some more examples made with this lens)