It depends greatly on where you live and how busy the contractors are in your area. I hung and finished drywall for years. If you use 4x12 sheets you will save on materials because you will only need 2 sheets per wall in the 12x12 room and 3 sheets for the ceiling. This is 11 sheets.
The 15x15 room will need 2 sheets per wall plus 6 feet and 3 sheets for the ceiling plus 3, 3 foot wide sheets at the butt ends. This is 15 sheets (extra scrap will remain).
26 sheets X's price per sheet, one 250 foot roll of paper joint tape, three (minimum) 5 gallon pails of joint compound, any outside corners will need metal corner bead, a large box of 1 3/8" (for new work only) drywall screws, 80 grit sandpaper to smooth out the compound, get a stick-type holder for your sandpaper. (the holders attach to any mop handle that are the screw type), one 5 gallon pail of drywall primer, and finally, your finish paint.
As you can see, there is a lot of material to purchase and heavy labor involved. I would have to pay a helper to assist me in putting up the sheets. I would charge you $70.00 per sheet complete. This works out to $70.00 x 26 = $1,820. Generally, you should expect it to be under $2,000.00.
Hope I helped :)