I've seen it done 2 ways the first is to retrofit the original soft top well, top frame and tonneau to your car which could cost you around $4,000 to $5,000 in parts plus another $2,500 to get someone to weld it all together.
Or I have seen where someone has added just the soft top frame etc which leaves it exposed when it folds down but it would be far cheaper. The top frames usually sell around $2,500 in decent shape and new canvas is around $500.
I think there was actually a Mercedes Bulletin on how to retrofit a soft top to a California Coupe