New costs of delay to the airline are presented for a range of aircraft and by phase of flight. It is shown how these costs may be suitable for cost optimisations during flight prioritisation decision-making. Desirable properties of prioritisation procedures are benchmarked against taxation principles. It is shown that Maximum Take-Off Weights, historically established as a charging mechanism in air transport, could actually be used in the context of determining flight prioritisations. Using non-linear delay costs is superior to the use of delay minutes in such optimisations and often produces more equitable solutions. Issues regarding the scale of solutions sought (e.g. by several flights or a whole network), and the implementation of truly dynamic cost optimisation, both require further research.