That is not exactly correct. Modern body armor mostly made of Kevlar and Titanium plates is standard issue for most modern armies and is not that expensive to manufacture. The Russian body armor is considered the best protection in the world and is being modernized further with the 21st century RATNIK (warrior) integrated combat gear. Soviet (now Russian) body armor tends to be based around
metal plates made of steel or titanium. The standard issue 6B23 and its predecessor, the 6B12, both utilize steel plates in Russian military service—although titanium and ceramic plates are also available. The cost of equipping a soldier with effective body armor, if produced locally is less than $5,000. In economics and mass manufacturing, it is called economies of scale as the cost of production goes down with number of units produced. The H&K G3 license manufactured in Iran since the late 60's is also now outdated. In the interim Iran has bought a few thousand Russian AK-103 but the whole Artesh basic assault rifle will need replacement as well. A lot of domestic prototypes have been shown lately but there has been no mass manufacturing.
So Iran can equip half a million men for the cost of a modern fighter jet. It is just that their is little or no value placed on the ordinary conscript. Perhaps now with senior officers exposure to modern militaries like Russia and China, this archaic thinking will change and more emphasis on manpower protection and professional development.
Turkey which is not as rich as Iran and has the second largest NATO army has equipped all its soldiers (who are also mostly conscripts) with NATO standard body armor and hence suffering far less casualties in Syria or against the war with Kurds.
Iranian military or Artesh as envisioned by the Shah was supposed to be equipped with the most modern gear, including body armor. It just makes common safety sense and morale enhancement to invest in providing soldiers with at least minimal amount of personal protection. Even simple things like ear drum protection gear at Home Depot cost less than $5. So the math is not that hard