Yea, it was a while ago, but I think we estimated the cost around $500 a year, and our average cost per developer at $100k per year.
IIRC we were paying around $20 per square foot, and the office sizes were small, like 8 by 8. So an extra 25 SF cost about $500 per year. You can make an office almost as small as a cubicle, the difference is you need space for a door to swing in, office walls are thicker than cubicle walls, and you need HVAC and power.
So there is extra expense in the build-out, but that's trivial over time if you are going to stay there a long while.
Edit: And even if you want 10 by 10 offices or my math is off, 1% a year is essentially trivial as well.
Its the same old Peopleware/Mythical Man month issues. Lets say you have 10 developers who each have an average throughput of X working as a team, but you need to double output. You double the size of the team, but get only 17X output because of the overhead of working together, so you add more developers until you end up with 25 devs providing 20X output.
What if instead you give everyone the latest fastest computers, individual offices, etc, and you make your devs 10% more productive? Now you only have increase the team size to 21 devs to get an output of 20x. Essentially you've saved 20% of your engineering payroll, over a half million dollars annually in todays dev costs, to achieve the same objective.
The costs of providing a good productive workspace is trivial in comparison to it's benefits, no matter how you measure it.