For ports, if they also give some defensive bonus i think 8 is good, and it also takes two turns to build, but that is only the first level and you can only tranport 1 ship there, for additional levles they cost 3 each and the maximum you can build in a teritory is the IPC value +4. Also ports have to border a specific sea zone even if the territory they are in borders multiple sea zones
for airfields, we dont have to dicuss them now but i understand they were much cheaper and more abundant than ports, but i was thinking that you pay one IPC to build an airfield in a territory and the number of airfeilds in a territory determines how many aircraft can be based there. And, if you capture an intact airfeild you can land planes there in combat or non combat.