USB powered Desk Fan vs AC powered Desk Fan
The answer to your question is NO.

First, there is no 12 V USB, at least in the normal desktop PC. The 5-V little toy fans are just toys.

Second, to get the USB port power, personal computers use AC-DC power supplies, ATX12V style or similar. They usually operate at 30% of max load, and efficiency is about 80% in that mode. So the conversion into USB power is only 80% efficient as compared to wall outlet.

Third, if you plan to convert 5V USB into 12-V fans, there will be another 20% loss in efficiency, not counting that you will likely consume all USB power budget and occupy all USB ports (due to current limits of USB connectors).

So the USB idea is really not flying.

What you need to do is to get a $30 AC wattmeter (Kill-A-Watt for example), few of them, and measure the actual power consumption of modern computers on your all 75 machines. Then submit the data to your management, and compare with your established power limits, accounting for cost of batteries, and don't forget the negative environmental impact of batteries disposal. I am sure the simple AC fans will win hands down.