Cost means less than effectiveness.
Compared to what? None of their neighbors have armor in the category of M1s except Saudi and Kuwait which do not have large enough armys to be a real threat.
You can have the best tanks in the world but, with so few, not be able to cover the 1000km border with Iran. Not to mention the border with Iran’s formal ally Syria. 140 M1s is 4 tank bns or 12-20km of frontage. To properly fill the IA out, they need 1400-2100 tanks. They cannot afford the delivery time or money spent on all M1s.
For the price of those 140 M1s, they could buy 1400 M60s. Then, as they have money available, upgrade them to M60-120S. The M60-120S cost half as much as the M1 and is better than anything Iran or Syria have. Simular to the upgrades Turkey and Jordan are going with.