So, the 335i sedan costs $1900 less than the 335i coupe, but lacks the fold down rear seats and sport suspension that the sedan has standard. One you add those two options to the sedan (which requires adding the sport package to both cars, which is prorated based on content), the price difference is less than $1000.
The M3 sedan costs $2700 less than the M3 coupe, but obviously has the same suspension setup. It probably only lacks the fold down seats. So I suppose the rest of the difference is the carbon fibre roof? But, I wonder if that also means you have to pay extra to get a sunroof on the sedan unlike on the coupe where its a no-cost option.