There are three rear spoilers for the GT350, including the one for the 'R'. The small deck spoiler, the Track Pack spoiler and the 'R' spoiler. You can go to FordParts.com and find the spoilers and their associated prices. The Track Pack spoiler which RobWidner attached is in fact the Track Pack spoiler. It retails for $535.71. However I have seen it on the internet at at least one dealership for $412.50 plus shipping. My dealer matched that price as well.
On the FordParts.com site, enter all of the info for your particular vehicle, which for me is 2016 Ford Mustang. Under 'BODY' select 'Aero Ground Effects', and then Shelby GT350. Then you will see the various spoiler breakdown with part numbers and prices. Good luck.