a base is soo much more than a harbour, it is all the industry, logistics and support you need to operate at maximum efficency. For pacific it means heavy convoy lines. for europe it probably also mean many rail lines and also alot of gaoline.
I like the prize at 15. it makes it expesive but worth it to build it in egypt and gibraltar.
In my games I have seen german airbase in yugo and an italian one in north italy (or is it south that starts without one)
it is also important for the dynamic, if all the german bombers are in italian controlled ukraine or rostov, an italian airbase there can make the allies less safe on the seas. that extra airbase might mean counterattack with all planes in the north sea or with bombers and finns in norway. Well worth the 15 IPC, but the italians have to save if they have to small income.