There is a set number of workers for each airport. If you would like to buy some more it costs you 45GPs for 5 workers (9GPs each). However, buying premium pass simply worth the GPs you have spend.
Take IAD for example, 110GPs for a premium pass, even if you igrone the worth of extra contracts, sliver planes, you still get 20 workers (5.5GPs each, significantly cheaper) at the end and that will help you more when you are doing sliver plane mining and it will also help to boost the process in this airport.
It is so extremely disappointing that the developers have taken away this opportunity to get GP by selling old planes! The goal of getting the Concorde at some point is extremely distant and it was my motivation to keep playing this game. Whether I keep up this motivation for 2 more years is questionable, there is not really much else that motivates you to keep playing when you have leveled all the airports.