When you feel like it's time, then purchase your homebase airport. Just be sure you have enough cash and credits though.
So this is my first SERIOUS game that I am determined to finish. Unfortunately, my hub is Seattle. It is worth buying?
You better buy a very crowded airport that has max used space and charge the users of the airport like hell to maximize profits
Is it likely that they will notice that the airport is private? I personally don't think so, since nobody goes to a full airport's menu to buy slots, thus they won't see the airport is private, but I may be wrong.
The only benefit from owning an airport is that you generate extra profits from slots, counters, offices etc. However, if you fail to upgrade it at the right moment, then the airport will be taken away from you. It's your choice.
How expensive is it to upgrade?
Depends on how popular the city is. The more popular, the more expensive it gets.
What do you think?