Two points:
First, as already mentioned, a carrier has with it, not just aircraft, crews and maintainers, but also radar, full command-and-control facilities, fuel, cafeteria catering, hospital, dental surgery, chapel, brothel etc. etc. (OK, so not the last, but you get what I mean). All these should be factored into the cost when debating ground vs seaborne air power.
Secondly, a carrier can do 'graduated response'. If you're going to up the diplomatic ante, flying a squadron of ground-based aircraft into the area with all it's support being flown in (something the RAF do very well), you're also making a pretty powerful statement. A carrier can just appear over the horizon and lurk offshore; "HMS Nonesuch is in the area having just finished major exercises with the Swiss Navy". Yeah, yeah, right. Nobody believes it, of course, but the diplomatic niceties are observed, and those who need to, get the message.