The estimate, included in a quarterly World Bank report on the Middle East and North Africa, was released on the same day that world leaders in London pledged more than $10 billion through 2020 to help the Syrians.
Some 260,000 people have been killed in the five-year conflict, which has forced 4.6 million people to leave the country, according to UN figures.
The Syrian war and its effect on Turkey, Lebanon, Jordan, Iraq and Egypt "have cost close to an estimated $35 billion in output," the World Bank said.
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The figure, measured in 2007 prices, is equivalent to Syria's 2007 GDP, the report said.
The influx "of more than 630,000 Syrian refugees have cost Jordan over $2.5 billion a year," the report said. "This amounts to six percent of GDP and one-fourth of government’s annual revenues."
All of Syria's neighbors "are facing tremendous budgetary pressure" because of the conflict, the report said.
"Unemployment is high among refugees, especially women and those who do work often work in the informal sector with no protection," said Shanta Devarajan, World Bank chief economist for the Middle East and North Africa.
"About 92 percent of Syrian refugees in Lebanon have no work contract and more than half of them work on a seasonal, weekly or daily basis at low wages."