"I think the London bombings, including buying the train tickets and conducting reconnaissance, were done for no more than 1,200" pounds, or about $2,000, independent defense and security expert Paul Beaver told The Associated Press.
Terrorism financing expert Loretta Napoleoni said the bombings were relatively inexpensive largely because the four bombers were homegrown, living with their families in towns north of London and traveling to their targets using a rental car and a train.
"They were just a group of friends who lived at home," she said. "Their only costs were the homemade explosives and the traveling costs to London."
The British Broadcasting Corp. reported Tuesday that police believe the attacks cost "no more than several hundred pounds."
Police have said the four used inexpensive, peroxide-based explosives. But they would not specify the estimated cost of the attack until they had completed the investigation into the "wider circumstances of how the attack was funded," police said Tuesday.
The BBC also reported that investigators believe teaching assistant Mohammad Sidique Khan, 30, was the group's chief's financier. He died in the attacks, along with the three other suspected bombers: Hasib Hussain, an unemployed 18-year-old; Lindsey Germaine, 19, whose occupation was unclear; and Shehzad Tanweer, 22, who helped out at his family's fish-and-chips shop.
Neither Beaver nor Napoleoni included in their estimates the costs of some suspects' travel to Pakistan, which has become a subject of the investigation.
Al Qaeda deputy leader Ayman al-Zawahri said in a statement broadcast in September that his terror network carried out the London bombings. Counterterrorism experts have not confirmed the link.
An August 2004 United Nations report said that al Qaeda had spent less than $50,000 on each of its major attacks since Sept. 11, 2001, including bombings in Bali, Madrid and Istanbul. Sanctions and other measures to block the group's funding had only "limited impact" on preventing attacks, the report said.