It is perhaps inevitable that contributors on a forum provided for aviation professionals
I can tell you, as one of the "professionals" more than a little involved with actually constructing a runway, that the figures being bandied around are equally incomprehensible to those of us who price up concrete, reinforcing bar and drainage pipes for a living. It certainly doesn't end up trickling down to the lads on the ground driving the excavators.
Although there's a lot more to it than the actual runway. Heathrow actually has a shortage of not only runway, but also aircraft stands, terminal space, access roads and railways, etc. However the way in which the figures get to these huge amounts, and whatever was quoted last time rises by way more than Spon's (the industry-standard pricebook of construction costs) inflation index tells you that, still, nobody has a real clue.
I understand that the figures include land purchase costs which get hiked by London-area house prices. But in actual fact, in Harmondsworth etc, Heathrow have (of course) been steadily buying up each house as it comes on to the market over the years, then just renting them out in the interim. Yet I understand these purchases already made are solemnly counted at projected forward prices.