That is not easy to estimate:
1
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.
2
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.
http://www.jpost.com/Middle-East/Intel-expert-Iran-protests-might-take-regimes-focus-off-Israel-522572
While the Iranian regime over-committed itself and was stretched way out, sanctions have slit open the soft underbelly of its economy. Now the regimes guts have dropped out, and it is bleeding rapidly. Likely a mortal wound, if we keep the pressure on.
It has no real allies in the region (neither Russia nor Syria stood with it when the Israelis struck). Its surrogate militias are hired help. It is hated at home, and holds power only with salaried goons (Basij). The core of its power (Pasdaran) are allowed to plunder the economy wholesale to buy their loyalty. When the money stops flowing, only a tiny insignificant cult really believes in their ideology. It is all built on mercenary self interest.
Although love for the regime depends on being paid, the hatred for the regime is real and deep. Tens of thousands of political prisoners have been hung - their families smolder with a desire for revenge. Their wealthy and powerful neighbors like Saudi Arabia, Israel, and the United States; have developed an existential enmity after years and decades of murderous attacks.
Their friends are rented, but their enemies have been bought and paid for.