As others have said, it probably depends, but it can be done pretty cheaply DIY.
I renovated a room in my house (sheetrocked in windows, added sconces, lighting zones, built riser, built half wall, built-in equipment rack with DVD storage, carpeted, painted, added chair rail, fabric below chair rail, replaced ceiling fan, projector shelf, etc.) all for a grand total of $2500. Took about 80 man hours over a 4-5 month period.
Add in my equipment and the total cost of the room was just over $10,000.