I see this argument get trotted out a lot. Compared to <dev salary cost>, adding service x,y and z percentage wise is just a small increment.
The problem that gets easily overlooked is that compared to the costs being made, the profit in many cases is also just a small percentage of the revenue, and the small % increment in costs can mean a large % decrement of profits. You should always compare incremental cost of revenue to the margin, not to the base cost.
Now if you are running a profitable SaaS business with 80% margins, a 4% increase in spend might not mean the end of the world even if it was wasted on spurious licenses or other frills, but if you are a healthy service business operating on 12% margins then that 4% increase will reduce your profits by a massive 33%.
This doesn't mean that you have to make do with the bare minimum and skimp on decent hardware or development environments, but it also means the marginal productivity/retention/quality increases must in the end be worth it in terms of increased aggregate profits, and not be justified as 'you already pay me 5K a month, so whats another 250 for ...