Adding a 3rd story to a 2 story home

[Jason Forman]

In areas like graduate hospital, I see people adding a 3rd story.  Trying to figure out how much that cost to see if it's worth it.   For example, I'm seeing renovated 2 stories go for low to mid 500s after repaired.  but the 3 stories go for the 700s.  

For a 1200 soft property, I estimate 130-140k in repairs for the 2 story method.  Everything needs to be updated aside from roof and maybe a bathroom.   

How much different would the 3 story addition typically cost?