Im a sales administrator in one of the uks leading Webhosting Companys, its a family business with my mother as the CEO. (never work for your mom!!)
a website of this size is one hell of alot of money!
Just now theres 400 people browsing these forums, thats alot of CPU processing and data retrieval to be sent back over the internet.
Firstly, a site of this caliber would require a dedicated server with the latest specs on a extremely fast line.
For me to host this website i would budget it around £3400 per year for first year, this includes the price of the dedicated server.
This would get the website running super sonic speed! it then depends on what forum based software you use.
If the pprune admin would like me to try get a decent quote for him with some money off, im more than willing to investigate for him :-)