Fighting the war on terror requires an enormous expenditure of money. Yet, conducting a terrorist operation can be surprisingly inexpensive, and inventive terrorists use every means at their disposal to raise funds and move money, from petty crime to bitcoins to legitimate charities.

While the Islamic State (IS) is considered the world’s largest and richest terrorist organization, terrorism now takes myriad forms. A number of recent and notable terrorist operations cost substantially less to implement than the Sept. 11 attacks, which were estimated to cost the perpetrators a mere half-million dollars.

The July 2005 transport bombing in London, for example, probably cost no more than $12,000. The Financial Action Task Force (FATF) on money laundering estimated the 2004 Madrid train bombings cost roughly $10,000. The Jan. 7 attack on Charlie Hebdo? Less than $5,000. Only $8,000 was required to carry out the horrific Paris attacks of Nov. 13.

Tracking such small amounts is akin to finding a needle in a haystack. But these small amounts add up, and this is where we need to be concerned. In 2014, the Belgian Financial Intelligence Processing Unit released a report that noted it intercepted 6.82 million euros ($7.49 million at the current exchange rate) in terrorist financing in 2014 alone. This amount represented an astonishing 165% increase from 2013, with major new sources being identified as Qatar and Kuwait. The report specifically noted that the amount has increased due to the number of terrorists who went to fight in Syria or Iraq.