Carriers by themselves may seem cheap but when you factor in the other unts that MUST be built with them, the whole group is very expensive. At a minimum you NEVER want to have a carrier without a destroyer because that makes it extremely vulnerable to sub attacks, and you may need a blocker to make a getaway.
In my mind, fleets come in two flavours:
Offensive: Carrier, fighter, tactical bomber, destroyer, cruiser, 2 subs. total cost is $69 not including the transports and ground units (5 transports, 1 armor, 4 artillery, 5 infantry will run you another $72).
Defensive: Carrier, 2 fighters, 2 destroyers, battleship. total cost is $72 not including additional destroyers or subs, and the naval/air base you want this to be parked next to (naval base, air base and 3 fighters costs another $60).
Bombers nearby completes either package.