This is actually a good price for this Nikon Fisheye-Nikkor 6mm f/5.6 circular fisheye lens with 220° angle of view: the current bid on eBay is $5,999 (sold by Adorama). At the last 29th Westlicht Camera Auction (June 2016) the same lens sold for €16,800 (around $18,880):
Additional information on this lens can be found here.
