Cost of vaulted/pitched vs flat in single story extension?

[Somethingtodo]

Any idea? - love the idea of making the roof vaulted in new single storey extension - but wondered how much more this will cost compared the flat?

eg 20% more or £x per m2 of footprint?

Our single storey section is pitched and vaulted - I know that having the ceiling vaulted rather than a suspended ceiling was no extra cost as we changed our mind half way through the build (and got a free Velux out of the builder) and the pitched roof didn't add significantly to our quote. But it's only a 3m by 3m section of a mostly two storey side and rear extension.

Thanks Teddy -- that is great to know...is so much nicer...............