The cost of the second runway was £172m. I would suggest it's paid for itself already !
You may wish to pop over to the Heathrow and Gatwick threads where the cost down there are estimated not in millions but "billions".
3billion at Gatwick and 6billion at Heathrow. Those are eye watering figures.
By comparison the Manchester expansion was peanuts and might by many observers accounts be a shrewd and very cost effective piece of investment.