The cheaper ones tend to take longer. I found this out when I was panicing in Pakistan as my visa was expiring and i had to leave but I didn't want to spend 100$ on the number. So I also applied for another number where you can pay once you arrive in Iran. About 4 months too late I received the number... in that time I used the expensive one and had traveled through Iran.
Just be careful - I meet a girl at the embassy who used a doggy one and kept her money and did nothing for the number. I would recommend using a company someone has used before.
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