I've done a lot of work in nonprofits, education, and government domains, and this can be a bit cumbersome. My experience is that in most cases, this comes down to administrative or bureaucratic issues which make sense on an organizational level, but very little sense on an individual contributor level.
At an organizational level, it's often not so much about money as about fairness, controls to avoid corruption, IT/supplier overhead costs, or similar sorts of reasons. I've been in C-suite, individual contributor, and everything in between. To flip things around, it's often cheaper to by a standard $3000 thing than a non-standard $1500 thing. Equipment costs are much lower than labor costs.
From my perspective, given three choices:
1. Spend $2000 of my time and $2000 of my boss' time to make an exception for a $500 monitor purchase
2. Be less productive
3. Drop $500
Often, I pick #3. I like my manager, and I don't want to complicate his life over a monitor. That lets me complicate his life over more important things, by the way.
When I was a director, I'd occasionally even spend personal dollars to buy things employees needed too. Dropping $20 is often easier than making a case to expense $20.