In the absence of the Russians, Iran has been forced to spend on defending Syria.
For a long time, Russia has been transferring a significant part of its missile defense systems from Syria to Ukraine to defend itself against the attacks of Ukrainian defenders during its invasion of that country. This occurred while Syria and its army in most cases had no hope other than these systems to create deterrence against Israeli attacks. Moreover, in many cases, the presence of systems like the S300 in Syria merely played a deterrent role, and the Syrians were unable to use them.
Reports indicate that Iran has transferred air defense systems to Syria. Part of what has been transferred includes systems known as Bavar 373, and another part comprises systems that Iran has purchased from other countries, specifically China. Last month, Syria successfully used some of these systems.
Israeli attacks in recent months have been unprecedented in intensity and success, and apparently, a number of Iranian officers have been among the casualties of Israeli airstrikes. Russia has even imposed the secondary costs of its invasion of Ukraine on Iran, and to defend Syria, Iran, which has practically lost much of its influence in that country, must bear the costs.