It has already wasted $150 billion given by Obama paying its debts to Russia for equipment, weapons.

Iran also pays for Russia's military campaign in Syria, fully funding Syrian regime, keeping tens of thousands of mercenaries there and another 100,000 mercenaries in Iraq. This amounts to tens of $billions a year.

In the Yemen war Iran supporting the Houthis requires many more $bilions annually.
Funding terrorism in Lebanon, Bahrain, Arabia, Hamas doesn't come cheap either.

Now Iran has to worry about interior security because what will be an ever increasing efforts by ISIS to harm the very center of the regime.

Couple this with the fact that oil prices are in a down spiral toward $40 per barrel and below when Iran needs at least $80 or more per barrel of oil to sustain such activities in the long run and to satisfy economic needs of its citizens and it is easy to foresee big troubles for Iran