I would not call the first example a "remodel". I'd call it an
additon. And I if adding that room means everything from the
foundation up, $8600 sounds very low. And the second example shows the
problem with a fixed ballpark cost, because remodeling a kitchen is
very different than remodeling a bedroom.
I would say $135/ft is in the ballpark for new construction of an
average two story 3000 sqft house, using typical grade materials that
you see in new subdivisions here in NJ. But that is for a whole house,
where it's easier to make an overall comparison, because there is a lot
in common. Even then, that number could probably drop to $115 on the
low end, or be taken to $200+ by adding more tile, custom woodwork,
high end appliances, energy efficient heating/cooling, special windows,
high end plumbing fixtures, etc....
Now, since the bedroom in your example is all new, why isn't $135 a
good number? I'd say it's going to be higher than that because first
it's a much smaller job than a whole new house. Second, it involves
more work dealing with tieing it into the existing structure, as
compared to starting with a clean slate.