The figures were released as Tony Blair announced his departure, leading to claims from the opposition that the government was "burying bad news".
The Tories and Lib Dems also claimed that the Home Office broke the law by releasing the updated figures a month later than they should have.
The Home Office put the £400m increased costs down to extra staff carrying out vetting and extra anti-fraud measures.
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With stirring oratory cast as a personal testament, he declared: "I ask you to accept one thing. Hand on heart I did what I thought was right. I may have been wrong. That's your call.
"But believe one thing: I did what I thought was right for our country.
"I have been prime minister of the country for just over 10 years," he said. "In this job, in the world today, I think that is long enough for me, but more especially for the country.
"Sometimes the only way you conquer the pull of power is to set it down.
Tony Blair today ruled out demands for a new inquiry into the July 7 bombings, denying claims that MI5 overlooked crucial evidence which could have prevented the attacks
In the Commons, the Prime Minister said a fresh inquiry would be a "mistake" and would undermine support for the security services in the fight against terrorism.
His comments came amid reports that MI5 did not show surveillance photographs of the bombers' ringleader Mohammed Sidique Khan to the original inquiry by the parliamentary Intelligence and Security Committee (ISC).
The BBC reported that although MI5 had six photographs of Khan taken during the investigation of another terror plot, only one was shown to the ISC.
