Companies typically buy laptops with an assumed 3 year lifespan; so, let's assume a high-end $3000 laptop, that would be ~$83/month. Of course, you need a computer to access Github Codespaces, so this isn't saving all of that money. Maybe companies can cut some costs by buying cheaper laptops?
Or if you want a more apples-to-apples; a Lenovo ThinkStation P620 runs ~$2800 for 32 threads and 64gb memory. DIY would also land somewhere around that $3K mark; a Threadripper 2950x (32 thread) is $1100; 64gb of memory is around $250.
This, of course, is the most expensive option they have; the more standard option would be 4/8; for which you would pay ~$85/mo (all the time, normal work hours), for the privilege of a machine that's far less powerful than any laptop any of our developers have. For god's sake, my M1 MacBook Air is an 8/16, with four of those cores far more powerful than any three year old Xeon Azure has running. And it was like $1400.
I understand maintaining dev machines isn't the easiest thing in the world. I have never once worked anywhere where it was such a problem that the company would justify tens of thousands of dollars in spend. Because it kind of is one-or-the-other; time invested in making Github Codespaces work for your company is time not invested in maintaining local dev environments. And Github's ideal end-state is companies totally rely on Codespaces, so the local dev environments languish.
Its not worth it.