For years Boeing could not sell aircraft parts to Iranian airlines because of sanctions.
Now that these specific sanctions have been removed Iran says it does not want to buy spare parts from Boeing. According to Sar Ghale, who is member of the Iranian parliament Construction Commission:
Boeing’s offered price for spare parts are three times more than current market prices
So basically its cheaper for Iran to buy these spare aircraft parts from the black market, rather than directly from Boeing.
Here we could have two scenarios:
1 – Boeing is using Iran’s isolation as an opportunity to charge exorbitant prices (as the Iranian MP suggests), or
2- Because Iran’s planes are so old and out of date, the costs of spare parts for them is genuinely higher.
One thing is for sure: the Rouhani government has taken the right decision to ban flights by Iran’s own locally assembled Ukrainian aircraft such as the IR140. These death traps were not certified to fly by the Khatami government, but the security apparatus of the regime which was assembling them decided to set up its own airline and fly them anyway.