It is cheaper to have an aircraft carrier, and a few escorts, in that it allows you to have an airbase anywhere in the world if you want one. That doesn't help all that much when dealing with land locked nations or locales (Tibet?) but since about 80 % of the world's population live within 200 miles of a coastline ...
What a carrier can't do is move big metal. (C-17, C-5, C-130, A330 ...) For that you have to have more runway. But, if you are dropping in to visit from the sea anyway, it is cheaper, albeit a bit slower, to bring stuff in a hull anyway.
