I believe in most cases (if not all) if you fit the RS clutch you need to match it with the RS flywheel too? Even replacing the standard clutch there's often wear found on the flywheel once it's stripped (or the flywheel itself has been slipping) so unfortunately you should be changing the pair as a set.
The difference in pricing for the RS clutch can't be down to it being new and under warranty, cos the same applies to the ST clutch for ST's under 3 years old. And ecconomies of scale 'should' apply to the ST clutch parts cos there's a hell of a lot more cars out there with it fitted (ST, Kuga, Mondeo, S-Max etc) compared to a max of 10,000 RS's if they even build that many.
PS anyone from Ford please ignore the above