Irix is selling “used” Irix 15mm f/2.4 Firefly lenses for Nikon F-mount for $299 on Amazon update: SOLD OUT (the already discounted price is $390). I was told that these are basically brand new lenses in dirty or slightly damaged boxes (not refurbished since they were never sold before – they come with a full warranty). The lenses are fulfilled by Amazon and a return should not be a problem in case there is an issue.
Previous coverage of the Irix 15mm f/2.4 lens can be found here.