JYP, we crossed a few days before you only...
Also, I emphasize enough how incomptetent/stupid/scammers were the Armenian officials both entering and leaving the country.
Expenses to be considered: a) "Ecologic and other concepts tax" on arrival (25000 Drams=48€ for our small car, reduced to 22500 after discussing why "brokerage fees"! Scammers) and b) another "obscure" tax (7000 Drams=14€) when leaving -no chance, "pay or stay" after talking to a big boss- to be paid in local currency. The trick: not having enough Drams, we changed in the bank office in the customs. Exchange rate = total ABUSE. Add 1 photocopy/1€ of documents you may not have copies of (was it Armenian ecologic tax?). Equation=scammers.
Russian military post (very friendly) will stamp your passport when exiting Armenia.
Getting to Iran, reaching heaven. But, if arriving late (2:30am for us), keep in mind that no gas station opened in Siad Rud (no hotel either) and next is Jolfa, so ca. 80 kms from the border to fill your surely almost empty tank thirsty of cheap Iranian gasoline.