They are a lot easier to destroy than you think!
Although more modern tanks have defensive counter measures they can deploy but at the end of the day those countermeasures are only good sometimes and usually against one missile or rocket not multiple projectiles
conclusion is at this day and age tanks are only good for urban warfare against insurgents not that they are completely obsolete but their mission by the most part has and will continue to changed to a more small command and control center used for communication, deployment of small UAV & UGV's, electronic warfare, etc
The day's of deploying large tank forces to win wars ended long time ago
Iraq had 5 times the Tank force of Iran and although they were successful in their initial invasion but at the end of the day they proved to be useless because Iran was able to push an armored division 5x it's size off most of it's territory within a year using fighter, helicopters and shoulder launched anti tank missiles and rockets.
If it was me I wouldn't buy more than 100 of these T-90's and I wouldn't pay a penny over $2 Million for them they can take it or leave it... Russia is building the Armada tanks and now they wanna fund their Armada fleet by selling their junk to Iran.
Best thing for Iran to do is to build all its armored division surrounding one chassis and one engine that can be adaptable to be used in APC, IFV, Light Tanks, Large UGV, MBT, etc and this will in the long run cut down costs of production and maintenance. This is exactly what the Russians plan to do with the Armada
Having swarms of armed unmanned ground and aerial vehicles is the future and if your not already working on mass producing these machines in large number than you are already behind