To have a professional install a new roof on a 1,200 square foot house costs from $4,000.00 to $7,000.00 which is a good price range for a pro install of a replacement roof.
That includes tearing off the old roof, cleaning up and installing the new roof and any flashing needed.
If you know how to roof the house you could probably buy the materials for around $3,000.00 but roofing is a lot of work and best done by a pro roofer who can get the job done safely and quickly.
A good roofing company can tear off and install a new roof within 1 day which is faster than I could ever do.