Silverstrata
In the spirit of cooperation may I point out that I think that Heathrow covers an area of nearer to 6,400 acres and not 3,000. Let's assume the site is 4 miles long (generous but based on 12,000ft long runways) and 2.5 miles wide ie 10 sq miles @ 640 acres per sq mile. Using your reasonable valuation of the site of £60 billion and an area of 3,000 acres, I make that £20,000,000 per acre. So lets multiply that by 6,400 and we arrive at a very reasonable valuation of £128 billion. Spend £50 billion on the new airport and we are left with a profit of £78 billion.
You have convinced me; of what I leave for you to work out.