Companies rarely give prices without a view of the job because the job can vary so much. Even access to the roof can make a huge difference. We have done a job where the roof was miles from an electric point and down a tiny path where everything had to be taken by wheel barrow. Needless to say the job was three times as expensive as if it had been next to the house on a driveway. If the roof is rotten and has to be replaced it makes a difference. If there are not enough joists in the roof causing it to sag a bit, it makes a difference. No sensible company will comit to a pice unless they can be sue they have coveed eveything.