Cost to rebuild from the studs? New construction?

[Christina Traffanstedt]

Does anybody have a ballpark idea of price per sqft for what it would cost to rehab a house that has to be taken down to the studs? Including demo, electrical, plumbing, walls, flooring, roof, etc. Basically just working with the existing foundation and what ever studs/framing can be salvaged on a house that has just sat vacant for however long. 

Similarly, what about new construction like adding more square footage onto an existing structure?