Report from 2018-06-19
Passports: 1 Ukraine (male)
Time before expiry: 3 days before visa expiry
Extension granted: 30 days from expiry date
Price: 34,500 toman (345,000 IRR) + 500 toman (5,000 IRR) for bureaucracy papers paid directly
Time spent: from 7 am to 8 am = 1 hour, pick up at 1 pm same day
The location and other reports are correct. The waiting room is spacious with seats, there is a toilet in the yard, bring your own drinking water. An officer in the waiting room (on the very right) named Javad is friendly and speaks good English, asked me just chit-chat questions, no problems even it being my 2nd Iranian visa this year (1st one was extended to maximum 3 months, and this is my 1st extension on the 2nd one, so starting my 5th month). They give you a printed pocket map in English showing where to find the Bank Melli and photocopy shop, so good customer support

For pick-up they asked me what about tomorrow, but I said I wanted to go someplace else, so they said they'd do it today by 1 pm. I mentioned I was supporting Iran in the World Cup, it was impressive enough.

No other travelers spotted, just residence applicants.
So far much better experience for extension than Tehran or Esfahan (for me).
On an off-topic, I find Snapp in Yazd very cheap and extremely friendly drivers, I took like five rides that day all around town to sight-see and avoid walking in scorching heat. So get Snapp on your phone and with minimal Farsi you'll be independent and mobile and not ripped off on taxi fares.
Good luck!