Cathedral Ceiling Modification

(not sure if this is the right forum but im sure there is a few contractors on here...) We are currently looking at a house plan that has 3 bedrooms upstairs, one situated directly over the livingroom. Were thinking of modifying the plan by eliminating that one room to open up the living room and add cathedral ceilings. My question is, would the cost saving associated with removing the one room (drywall, ducts, electrical, flooring etc.) offset the cost of the more expensive joists needed for cathedral ceilings? What are some other financial pros/cons of doing this, (higher heating costs? lower property taxes due to reduced sqr footage, etc.?) -thanks