If devs salaries are so crazy its quite the opposite. NOT investing 1-1.5k/mo to improve their productivity by a measurable amount would quite literally be just plain stupid and I would question your boss ability to think critically.

Not to mention - while I know many don't like it, they may be able to achieve enough of a productivity boost to not require hiring as many of those crazy salaried devs.

Its literally a no-brainer. Thinking about it from just the individual cost factor is too simplified a view.