The biggest advantage of a drone is endurance. As everyone know they can stay up a long time. They have a lot of disadvantages too.

They tend to crash a lot. The article David provided stated that 1 of 6 British Reapers in operation since 2008 has crashed. That is about par from what I read.

They are not as cheap as people like to think. A Reaper is 10 million and up and a Predator variant is 4.5 and up. That is a lot of money, especially when they crash a lot.

They may do what they are told but sometimes they don't. This is one reason manned aircraft don't like to fly too close to them. That makes a difference when there are a lot of aircraft over a target.

They are slow, very very slow, a least the Predators are. If they are working an area and something critical happens 60-80 miles away, it will take them a long time to get there. If there is a strong headwind, forget it. The Reaper is a lot faster but it is a lot more expensive and they crash a lot.

They may not take up ramp space like a P-3 but the Predator isn't that small and the Reaper is a big airplane. You can't just put them in a corner. They will require ramp space, taxiways and a smooth runway like most planes.

In order to get that endurance, you have to have a pretty lightly loaded wing. This, I think, will give you problems in heavy weather. An F-104 rode out the bumps better than a Cessna 150. The same probably applies to the drones.

I don't know what their crosswind restrictions are but I would be surprised if they matched those of most manned aircraft.

Their absolute biggest disadvantage is one I don't often, if at all, see addressed. They are controlled by radio. If you can control them from far away, couldn't some real smart guy on the other side overpower your signal and take your drone from you? I know it is hard to do, but can somebody truthfully tell me it is impossible.

On the cost question, comparing a drone to a P-3 or such is a false comparison. You should be comparing it to a civilian light fixed wing aircraft like a King Air or a Caravan. Those airplanes new are rather cheaper to buy and used ones are way cheaper. They have fair endurance and if you put in extra fuel tanks they can have good endurance. They don't crash much and they can fly in heavier weather and probably greater crosswinds.

Even a Caravan is faster than a Predator and a King Air is as fast as a Reaper and much cheaper. If called to go support a situation 80 miles away, a King Air can get there in about 20-25 minutes. Civilian manned aircraft last for decades. I doubt anybody will be flying a 2010 model Reaper in 2040.

If you wanted a manned airplane that will approach the endurance of a drone take an old regional airliner like an ATR-42, which unmodified can have up to 10 hours endurance, and modify it with some internal fuel tanks. Then you would have an manned airplane with that would would approach the practical endurance of a drone without the disadvantages. You could do the same thing with a Dash-8 or a Saab 340. You would however, have to avoid overloading it with sensors.

You can also fly these airplanes with civilian crews which would be much cheaper.

There are alternatives to drones out there that can do aspects of the job better.