I see a
Refurbished Canon EF 300/2.8L IS USM at Adorama for $4000. Refurbished lenses are usually new lenses which have been returned to Canon for some reason. They are checked out and if there is anything wrong with them they are repaired and brought up to 100% new condition. The big difference between a refurbished lens and a new lens is that the refurbished lens will probably come with a store warranty, which means if you have a problem you send it back to the store for repair rather than to Canon.
The grey market lens is
$4200. Grey market means that it has probably been imported from overseas by the store rather than purchased from Canon. Canon normally honor a 1 year warranty on such lenses though they have no legal obligation to do so. Some other companies (Nikon comes to mind) do not warranty grey market items so they have to be returned to the store for repair. If you are uncertain about the warranty coverage, call Canon and ask them. They may also have some comments on the warranty status of refurbished lenses. I think they typically offer 90 days on refurbs (and the vendor continues that for 1 year).
The new, USA version is
$4340. This is a lens supplied by Canon USA to the store and carries the full Canon 1 year warranty, so if there is a problem you send it back to the Canon repair facility (either in CA or NJ).
All three lenses
should be the same quality and
should all fully meet all Canon's specifications for the lens. There is only one factory which makes them.
If you decide to save and go with the refurbished lens, you might want to double check that it's a
Canon factory refurbished lens (i.e. refurbished by Canon themselves). Adorama should be able to provide that information.
I see Adorama also have a
Used 300/2.8L IS USM for $3549.