The London bombings last July which killed 52 commuters cost no more than several hundred pounds to carry out, the BBC said on Tuesday after an investigation.
Economist Loretta Napoleoni, an expert on financing terrorism, told the BBC World Service the cost fitted a bigger pattern.
"If you look at 9/11, which cost only $500,000 to execute, and then you look at all the subsequent attacks that have taken place -- going from Bali to Istanbul to Madrid to London -- we actually see that the cost of the attacks is decreasing," she said.
Four young Britons detonated homemade bombs concealed in rucksacks on three underground trains and a bus during rush hour on July 7, 2005, killing 52 people and injuring hundreds.
British police said on Tuesday they would not comment on the BBC report.
The Madrid bomb attacks in March 2004 killed 191 people and wounded some 1,900 when bombs went off on four packed commuter trains.
Investigators at the time said Islamic militants recruited common criminals and used money raised from selling drugs to fund the bombings. EU officials estimated they cost less than 10,000 euros.
The BBC said the police believed one of the London bombers, Mohammad Sidique Khan, was the principal backer of the attacks and that he gave money to the other men to buy materials.
{{/usCountry}}The BBC said the police believed one of the London bombers, Mohammad Sidique Khan, was the principal backer of the attacks and that he gave money to the other men to buy materials.
{{/usCountry}}Douglas Greenburg, who studied the financing of the US attacks as part of the 9/11 commission, told the BBC the relatively low cost made it harder for banks to spot any financial irregularity.
"If you have someone who is working and depositing their pay cheques into the bank, and periodically withdrawing money and at night buying components for a bomb, constructing a bomb in their basement, what's the bank going to do about that?"