alosaurus, you've already heard from an expert that a minimum of three people would be required - and I agree with that.
So, that's £150k per team, plus allowances/per diems, accommodation, training, cost of lost revenue seats, etc etc - say a further £100k or so. So per team, you're looking at £250,000. You need one team per crew - I'll use a figure of five crews per aircraft (you use however many crews per aircraft your airline has), so that's £1.25m per aircraft. In the case of an airline like BA with 200+ aircraft, that's over quarter of a billion pounds added to the overhead. How many airlines do you know of that are making sufficient profit to pay that sort of cost? And don't you think that they are trying to maximise revenue already - so it's not as easy as you think to "just add on a pound or two per ticket"!
The old saying prevention is better than cure applies here. Security on the ground has to be improved to the point that nothing can get through - and that's another cost that's going to be passed on to the airlines and from them to the diminishing numbers of passengers prepared to check in three hours in advance of a one hour flight.
You asked whether there have been instances where the Skymarshal approach has failed: the answer is yes, many times - I can think of instances in Iran, Russia and China where shootouts have led to the aircraft going down.
SeaSunSandFly - interesting perspective, but I'd refer you to some of the air rage threads earlier where it was pointed out that a pilot's job - and priority - is to fly an aircraft. It takes a particular mindset to be able to use a firearm on another human deing - no matter what's going on - and having been in that position myself I can say that it's not as easy as just pulling a trigger - no matter what the movies might show! Some people certainly could do it - but everyone? And how would the crew act in the event that a passenger or member of the cabin crew is being used as a human shield - would they hand over their weapons or see that person killed before their eyes? A sobering thought.
Nor is there much room for manoeveur on most flight decks - one exception being the L1011!
Whilst El Al's on-board security team might be an effective deterrent (all of whom are in plain clothes and spread around the aircraft, btw) I'd attribute the lack of any occurrences on board their aircraft to vigilant and exhaustive ground security checks - which corroborates my point above.