Well figure it would be a 2 day project. Drywall, tape, mud one day. Sand, paint and trim the next. Labor and materials ?
JB is it an addition, or a room added on inside that already has a ceiling, or is that a part of the drywall ? ? What type of finish are you getting on the drywall, flat or textured ?
If they're only doing the walls - you'd have two walls 10x8(assuming the ceiling is 8' high) which means you'd have 160 sq ft to cover. The two 12x8, giving you 192 sq ft to cover. Total of 352 sq ft; divide that by 32 sq ft(4x8 sheet of drywall) and you get 11. So you'll need 11 sheets of drywall to cover that area, I'd get 12 or 13 to be safe, incase of any oopsies.
Reason I'm saying is maybe you'll want to invest in some of the materials, and contract someone to do the labor and it'll be cheaper in the long run for you.
I recently drywalled a room inside of a house. Framing was done, and it was also 10x12(x8) for a nursery. Took about 5 hours by myself, and then taped and mud took another hour or so. Sanded and painted the next day.