I'm using a mansard style roof for the 3rd floor of Highgate apartments to reduce the appearance of height and make it look like 2 stories plus a loft.
Mine is a modern interpretation - not provincial - with balconies jutting out in a cubic way rather than dormer windows.
I would expect that a mansard roof would cost more as it's a bit more complicated than a skillion or std gable especially if you put in dormers.
However if you are using it to gain more space then the increase in value will pay for it.