On a side note: from all the pictures of Gorilla Kingdom I've seen, it looks like a "£5.whatever million pound exhibit" to me. Gorilla holding is expensive.
It's a not-especially big island and a not-especially big house. Most of the side exhibits are either basic mesh constructions or very slightly altered exhibits that were already there from the old Sobell Pavilions.
I'd be quite prepared to believe there were logistical reasons for the cost (drainage works, that kind of thing) that you don't see in the final product but which have bumped the cost up, but I don't think the final product looks like it cost the money, and that's one of the reasons it gets stick. Chester's £3.7m Orang house and Edinburgh's £5.65m Chimp house (yes I did look those up!) are far superior, and both are much more impressive and animal-friendly structures for comparable, or less, money.