I looked into this earlier in the year.
I decided against it after finding out we had these kind of beams in the middle so it's far more expensive to do.
All I would suggest is phone up local builders alongside the loft conversion companies and see what the quote differences are.
I've seen a simple one done for just over £5k to one's that cost 30k
https://lh5.googleusercontent.com/-TEQksuy417A/UEc_5n5_3bI/AAAAAAAAAHs/Utd0m9ZJIrE/s720/roof%2520wall1.JPG