I am a property valuer although have never valued an airport and don't wish to start now!
However there are a number of approaches to arrive at a capital valuation: -
Rental - You basically capitalise the rental stream - unlikely to give a particularly high valuation for Manston!
Profits - You basically capitalise the profits -------- hmmmm!
Contractors - You calculate the cost of rebuilding the facility - Likely to be pretty high!
Development value - You work out a site value for alternative use
Or you could do part and part on the ground some areas have development value, some areas are occupied and achiving a rental stream.
Valuation is an art not a science and 2 valuers will come up with the same answer! But something like Manston is very difficult to value and I am glad I don't have the job.
In reality it is worth what someone is prepared to pay for it and at the end of the day that is a commercial decision by the purchaser!