Also check the joist size/spacing of the floor of the attic

A lot of times, the joist size and spacing of an attic do not satisfy the requirement for living space.  That means more cost for the joist and floor.
Even with the additional insulation to the roof, the attic may still too hot/cold (depend on where you live).  
My guess is there would be no GC would want to take the job for less than $20,000 or even more.  

Good luck!