Hi Rory -
The hardtop adds value to the car - a future buyer would probably select and pay for a 230SL with a hardtop than without - I know that would.
Two other considerations: If you live in a colder climate, and drive the car in the winter - the hardtop holds in the heat, and secondly, the hardtop changes the 'look' of the car - when I put it on in the fall, it's like I have a different car.
Depending on condition - hardtops go from about US $1500 to $2500 - some less and some more. One other point when looking for a hardtop - make sure the headliner is in decent shape. Replacing the headliner is difficult and expensive!! I have a 'rolling stand' (about $100) to store the hardtop vertically, so it doesn't take up much space in my garage.