So many variables with a pop-top that I wouldn't even pretend to guess. I'm sure many contractors here on Hometalk and elsewhere would be happy to come offer you an estimate, though.
As Steve said this can be all over the map...remodel "Ballpark" numbers are often quoted around 80-100 per sq ft for extensive remodels and 150 to 200 per sq ft for "new" additions. Again these are "ballpark" figures. Each local area will have there own ups and downs. The best way to get some better ideas would be to bring in some pros to get a more "detailed" idea of what you might be thinking about.
Lezlie, you definitely want a contractor to come out and give you an estimate. Most contractors should give you a free estimate so you aren't locked into anything. Also, they will be able to point out any potential issues and ask specific questions that will help them give you a more definite price. If you need a contractor, let me know - we have one in the area who does offer free estimates or you can come see us at the Southern Ideal Home Show this weekend at the NC Fairgrounds.