You think it is normal to pay so much for ventilation to install 70 S9?
Well, your 20,000 CFM number is in the right ballpark. Not sure why you were caught unaware of the cooling needs - there are many MANY threads on that topic.
Regarding cost, no, $17,000 seems excessive - unless the warehouse owner is just ripping you off for the right to modify their warehouse. Or your doing this on the East Coast of USA or California or maybe Chicago.
Remember that what goes out must come in. For S9s, I recommend about 1 square foot of screened opening per miner, so the equivalent of a 7 foot by 10 foot screened in area.
Absolutely layout your miners in a hot aisle/cold aisle arrangement - you do NOT want any exhaust air from the back of the miners mixing with their intake air.
I do hope your in an area with cool outside air.