On the excellent Accidental Tech Podcast there's a running conversation about Marco Arment's (Tumblr, Instapaper) switch to Go, from a much loved PHP, to implement feed crawling for Overcast, his popular podcasting app for the iPhone.
In Episode 101 (at about 1:10) Marco said he halved the number of servers used for crawling feeds by switching to Go. The total savings was a few hundred dollars a month in server costs.
Why? Feed crawling requires lots of parallel networking requests and PHP is bad at that sort of thing, while Go is good at it.
Amazingly, Marco wrote an article on how much Overcast earned in 2014. It earned $164,000 after Apple's 30%, but before other expenses. At this revenue level the savings, while not huge in absolute terms given the traffic of some other products Marco has worked on, was a good return on programming effort.
How much effort? It took about two months to rewrite and debug the feed crawlers. In addition, lots of supporting infrastructure that tied into the crawling system had to be created, like the logging infrastructure, the infrastructure that says when a feed was last crawled, monitoring delays, knowing if there's queue congestion, and forcing a feed to be crawled immediately.
So while the development costs were high up front, as Overcast grows the savings will also grow over time as efficient code on fast servers can absorb more load without spinning up more servers.
Lots of good lessons here, especially for the lone developer: