That is not easy to estimate:
Some estimates put the total cost of Iran’s support for the Bashar Al Assad dictatorship as running at $35 billion a year. Add to this the backing for Al Houthi rebels in Yemen, Hezbollah in Lebanon, as well as a range of other terror groups such as Hamas, and it becomes clear that the ayatollahs are paying a heavy price for their commitment to exporting their Islamic revolution throughout the Middle East.
Israeli officials have accused Iran of growing involvement in the Hamas-run enclave, and while Tehran froze its financial support to Hamas in the Gaza Strip after the group refused to support the Assad regime in 2012, it is now reported to be providing the militant group some $60 to $70 million.
According to Israel, two years after the nuclear deal was signed, the Islamic Republic has also increased its financial support for Hezbollah to $800m. a year.
When sanctions were in place Iran provided it’s Shiite Lebanese proxy group some $200m. Iran is also reported to be spending hundreds of millions of dollars in for their militias in Syria and Iraq as well as supporting Houthi rebels in Yemen fighting pro-government forces backed by a Saudi-led coalition.
Iran’s military budget is reported to be around $15 billion a year and its role in Syria has cost the country a great deal. In 2015 the UN special envoy for Syria, Steffan de Mistura estimated that Tehran was bankrolling embattled Syrian President Bashar Assad to the tune of $6b. to $35b. per year.