Couple of thoughts from me on this;
Generally, whilst RHD production numbers are smaller (making them 'rarer'), so too is the RHD buyer base. All other things equal, assuming manufacturers sized their production split (limited or otherwise) proportionate to expected demand, there ought NOT to be a significant price differential.
In practice, demand in the LHD and RHD countries drives the basis in pricing. Over the past 5yrs or so, we have typically seen RHD more expensive than LHD for many high end / limited edition marques: 458 Speciale / 430 Scuderia / 360 CS / special porkers --> all typically being more expensive in RHD guise.
My view is that in general a stronger UK economy and/or possibly stronger collector cult over in UK has driven much of that higher price base for RHD collectables.
So why the RHD CSLs cheaper than their LHD sisters? 
Could be that whilst there are approx. 2.5x the number of LHD CSLs available, there's a far greater following/awareness of how special they are over on the continent?
From what I have seen the pricing gap (whilst still
sizable) may have narrowed a little over last 6-8months, with most RHD CSLs sub 25k miles looking to sell £5-10k above original list price. which I don't think was the case in the UK a year ago.
Just my thoughts. Other views welcomed!