How to improve productivity with money?

A few weeks ago I've read yet another blog post about productivity. The author claims that deploying a third monitor onto your desk does not increase your productivity anymore (in contrast to adding a second screen). Even worse, he argued that adding the third screen will decrease your productivity. Business people would call this negative marginal profit. There has been an intense discussion on hn whether this claim is justified or not and this shouldn't be the place to continue the primarily opinion based discussion.

However, this discussion led to my question: How can you improve your productivity by spending money? Have you ever thought about how to spend your money efficiently in order to increase your productivity? Or have you ever bought something to increase your productivity and finally decreased it with that move? Please let me know!