Money no object then, Frank? Is it 'common sense' to pay two hundred pounds for a pint of milk in your house?
No, get it for around £0.50, give or take.
The cost of a tunnel for the M25 (if that's what you mean) is peanuts compared to the amounts the government p**ses up against the wall. HS2 anyone? Contributions to the EU? The millions wasted on mostly useless overpaid bureaucrats and "management consultants"? Could go on, but won't bore other readers any further.
Time machine or crystal ball? Which did you use in determining this?
Both, and some common sense. It's a political decision, not a financial one.
Of course with a third rwy, more carriers can operate to/from LHR because of lower costs and less congestion, and LPL can get a link to LHR. There, now we're back on topic!