I was talking with a friend who works at a company HQ'd in NYC and he said that the average cost per person/desk was over 60k/yr. It was a lot higher than I was expecting.
Of course there is other overhead, but space itself is in that range. So the max savings, in the most expensive market in the US, is under $10k per year.
Practically speaking, most people are treating the shift as an implicit agreement that employees can work from home 90+% of the time.
https://www.wired.com/2000/05/is-your-cubicle-shrinking-too/