Dormer costs in mid cape cod area?

Can someone give me a general calculation for an addition to a second floor attic area. House us 45' x 24' dormer would be just in back - no front small window dormers. Roughly working on half the attic like 45' x 12'. Nothing fancy - basic 2 rooms and a master bathroom. Just want to get a general idea of cost per sq. ft. Can anyone help me with this - Thanks so much!