Evan,
That's a sharp looking building. I am guessing, its 50 ft. with the carport overhang. I remember concrete being $65/sq. ft, when I did mine. I think it went up to $75-$80 over the winter months. That was about 5 years ago. I biult with 2x4's, 16 on center (OC). I might think of going with 2x6's and 24 OC. That is what the codes around me allow. The 2x6's would allow thicker insulation. I have a 10 pitch roof. It really puts a strain on your legs, putting the roof sheeting on. I also used a beam and joist. I think overall, that added greatly to the cost, and time. I have about 460 sq ft on the second floor. Also, watch on what size windows you use on the 2nd floor. Mine were just to small. So, I couldn't use it as living space. Therefore no closets.
I used 3D architectual shingles on mine. They were about $25-$30 per bundle or square. I think regualar shingles were $12-$15 at the time. When Katrina hit and gas prices went up, regular shingles were $25-30 per bundle or square. I remember $1600 for shingles, tar paper etc, and $2000 for labor.
$3600 for the slab, was labor and cememt. The $4500 quote was also for labor and cememt. It they use boards to form the slab, make shure they pull those boards out and take them. Termites like bauried boards for food. Next step is your garage.
Tom