Although moving around the tube network costs a small fortune with the recent price hike making ours probably the most expensive in the world it apparently takes only a few hundred quid to blow large chunks of it up:
An investigation by the BBC World Service into the cost of the London bombings last July has revealed that they cost no more than several hundred pounds to carry out... Officers now believe that Mohammad Sidique Khan, who worked as a teaching assistant, was the principal backer of the attacks and that he gave money to the other men to buy some of the materials.
So much for the theory of al-Qaeda masterminding the attack and providing financial backing. Looks like the whole thing was pulled off on a teaching assistant's salary.
That such a serious attack was carried out so easily and for so little financial cost should at least bring a pause to any further talk of metal detectors at tube stops - this kind of terrorism needs only a belief system and a target - in a city of seven and a half million simply gumming up the transport system is not going to be an effective deterrent.
Alternatively we can continue to make the city too expensive to live in or for tourists to come and vist and then we'll easily spot the badguys.
If you're really worried about terrorists we suggest hiding out at the London Dungeon - at fifteen quid a pop it's not only a horrible waste of money, but is also well out of reach of bombers on a budget.