Whereas the insulation should not be the major cost factor, with a 22 degree roof pitch you have 0.7 sqm more per meter for a 9 m wide house. More expensive is then the more complex roof construction, possibly you still need load-bearing walls etc.
Even more so some many working hours, scaffolding for the interior, hazard allowances and more residential drywall :)
The roof construction too, so in your case, that is the topic here. For us, the construction remained the same. Just wanted to point out that the 15000 for the gallery and co include far more than the roof construction itself.