The road taxes exceeding the costs for Visa
A visa for Armenia up to 21 days can be bought for USD 30 also directly at the Georgian-Armenian Border at Sadakhlo. For the car (Minibus 8 seats) I paid a road tax of 80 USD to the Armenian officials at the entrance to Armenia and about 22 USD at the exit of Armenia, also to the Armenian officials. (The car is registred in Georgia.)
So be patient to keep some money in your pocket when leaving Armenia.