I can't help with the costs, except to state that they will be *way* higher
than first anticipated :-) I am just finishing up this same type of project on
my house. In my case I only expanded the back third of the house into a two
story portion, although I completely razed the existing hip roof structure on
the remaining part and rebuilt with a much higher pitched gable roof to look
better against the new two story part. So my house now has a split level look.

In addition to all the good comments, I might add one more that may not be
obvious. Your present ceiling joists will not work at all for the floor
structure for the second story. So the entire ceiling structure will have to
come off in addition to the present roof. In other words, for a while you will
be living in a house with just walls, nothing on top but the sky. You could
leave the old ceiling structure in place and add new joists on top, but that
raises the height of everything.

--
Dennis