I'd rather pay for an air marshal to spend the duration of the flight in the jumpseat...
An armed air marshal, you mean? And who would prevent a "loco" air marshal from shooting the pilots... another air marshal?
...and what would a marshal do to prevent a pilot from putting an aircraft into a spin? Or shutting off all engines? Or selecting full flaps at MACH 0.81/FL380? Another pilot?
Its a catch-22...
Let's stop the hysteria. The immediate reasonable thing to do is to bring a cabin crew member to the cockpit while one of the pilots is temporary out of the flight deck.