cost to enclose a loft area?

Our family is expanding, and we’re going to convert a loft area into a bedroom. The project will require two new walls. One will be a full wall (approximately 9′ x 9′) with a door. The other is 11′ x 9′, and it’s already built up to about 3′ 6″ high (a half-wall that looks out over the room below — maybe that existing part of the wall can remain and be added to?) We don’t need to add electric or anything to either wall, but I am interested in insulation for noise reduction. Can anyone provide a ballpark figure for what this should cost? Thanks very much.