What's funny is that I actually have no idea what a non-open office would look like. I've worked for 15 years and have never even walked in an office that wasn't open. How do you give each person an office without using like 5x the space as a cubicle farm? That's the real reason that companies like open offices - because they can jam a lot more people in the same space. Someone who is ok with a 6x6 cubicle would feel trapped in a 6x6 office.
I worked in office space in NYC a few years ago that was $80/sqft. 6x6 cube is $3k/year. Using your assumption that an office is 5x bigger (not sure if that's accurate or not), you're looking at around $15k/year for an office.
So the delta is $12k/year, equivalent to 160-240 hours/year or 8-16% of a salary. IMO, with many developers you'll regain productivity far in excess of that by shelling out for an office.
The reasons companies eschew offices has more to do with taxes and lack of giving a hoot than cost.
Four employees in one 10x10 office works pretty well if you flex time and remote work so heavily that for all intents and purposes its a private office most of the time. In theory all four people might be sitting in there around lunchtime or adjacent to staff meeting time, but not most of the time.
That is 100 sq ft per 4 employees whereas giving each employee a tiny 6x6 cube (although I live in a 8x8) would be 144 sq ft per 4 employees, so if you go with offices you actually use less space than cubicles and if you are flex timing it the employees feel it is much more spacious.
I currently work in a team of 4 flex timers each with 8x8 cubes, in a sea of cubes, and I'd much rather have a nice 10x10 with a door for the 4 of us. We'd save a huge amount of real estate this way and get more work done.
You can of course fit far more than 4 employees in a 10x10 office if you ultra-flex time it. More than two decades ago I was baby sitting a large IBM financial mainframe network (SDLC, source route bridging, that kind of thing) and we had like ten people assigned to a 10x10 but due to 24x5 coverage, not to mention remote work, we never had more than two people in the office at a time.
I've never in my life so far, worked a job where everyone works precisely 9-5 M-F so I can see where non-flex time lifestyles might be more difficult. I would have a serious problem tolerating rush hour driving twice a day, five times a week, but thats another matter.
The place I work at had just recently (a couple years ago) moved from an older office with 6 ft cubical walls, to an open office with the short cubical walls. You can see over the top of them when sitting down, but they also have a frosted glass piece (1 ft high) stuck on top to maintain privacy (but still let light through).
Big thing I notice -- in the old office, it was pretty noisy, as it was all too easy for people to think they had privacy when they were in a high-walled cubical. The new office is a bit quieter, as people tend to talk in hushed voices more, since they can see everyone around them that would otherwise hear them.
