Personally I don't think it's worth that price and regarding the £1500 build cost, I can believe it given the work was supposedly completed by KoA. The build valuation probably includes labour although looking at KoA's updated page and pricing structure, however, upon closer review I'd question the build cost in lieu of other packages but without historical data it's hard to judge. If I was a buyer I'd also want to see the invoice from KoA just to see what they actually did and to confirm it was done by them.
On a side note, I'm not really sure why the cathedral/church photo was included... unless it comes with the gun in which case maybe £900 is an absolute bargain? Given the images are meant to represent what you get?