Costs to enclose a balcony?

[Steve Wang]

I am thinking of enclosing a 3rd floor balcony, turning it into a space which could be used for a home office.  The balcony is already surrounded on three sides, with the fourth (open) side flush with the rest of the house's exterior wall.  It is also covered by the roof and attic, and lies above parts of the interior 2nd floor.  In other words, the balcony is a concave recess into what otherwise would be an entirely vertical facade of the house.  

I'd like to open up the back wall of the balcony (leaving a railing or half-wall of some sort in its place) and put up a new exterior wall (with 1-2 windows) that is flush with the rest of the house.  (I don't particularly care if the new exterior doesn't quite match the older parts of the exterior, or if there are "scars" where they meet - almost nobody will see it from the outside.)  The balcony itself is about 6.5 feet deep by 12 feet wide, and is currently lined with the same vinyl siding as the rest of the back of the house; the side walls and ceiling will remain where they are but the vinyl would be replaced with normal drywall.  Laminate hardwood flooring, electrical outlets, HVAC duct (wouldn't travel that far, through the attic) would need to be installed.  

Unfortunately I'm not in a position to bring contractors out to look at the property right now.  Roughly how much would you think it would cost if I contracted all this work out?  Are there any parts that I should really try to do myself, if I have time (I'm pretty green at DIY)?  Thanks for your help!