I find it surprising that all the places listed in "roof contractor"
don't list approximates for full roof replacements, but only the
surfaces. *shrug*

What you're talking about doing is building a 1-story,
1000 sqft cottage. It just happens that you're building
it on top of your house. It's not a roofing job, it's
carpentry/remodeling. In any case, the price you
get will vary wildly, depending on exactly what you decide to
do, and who you get to do it, and what, if any adjacent bits
of roof/wall there are to cope with, how hard your building
is to get to, whether you want the house to look decent when
you're done, and how much you **** off your contracter.

You aren't going to get meaningful prices by waving your
hands. At the very least, we'd need a picture of your
house from the outside, (front and back), and a picture
of the unfinished attic space, pictures of your neighbor's
houses, if any are visible, and a diagram with measurements
of the rafters, beams (if any) and where the doors are
supposed to go. And a copy of any relevent local
building codes, including height/story restrictions.

--Goedjn