Let me give you a hint - if the existence of snacks made people work a mere 20 minutes more a week it would’ve already paid for itself. The calculus is a bit more complicated for full service lunch, but you get the idea.
For example I work in a company where the salary is pretty good, but they're also super stingy in general. No snacks, no socials, shitty laptop, etc. It betrays a lack of interest in making employees happy.
The absence of things like snacks is also an indication of how much the bean counters are in charge, because as you say it's probably good value for money in terms of morale, but as soon as the bean counters come in they see you spending £1k/month on snacks or coffee or whatever and that's an easy thing to cut.
1: https://steveblank.com/2009/12/21/the-elves-leave-middle-ear...
I worked at place before that didn't have coffee so people would walk to the coffee place in groups that took 15-30 minutes. Sometimes twice a day. It became a routine part of the day, everyday.
Like literally throwing the cost of an invisible bag of Starbucks beans in the garbage everyday to save the visible cost of brewing a few cups.