I have to ask. What kind of airline?
You need permission (which is bought) to use runways, you need an airplane (easily 50m), you need staff (maintenance, pilot, on-flight crew, lawyer etc.)
I'd say to just manage and fly 1 Boeing 767 is $60.000.000 investment plus at least $100.000 a month.
If you just want to own a hangar and have cropdusters and Sessna's stationed in it, basically renting out parking space to small aircraft owners. You can be done for like $3m and $10k/m