I consider perks as a part of the salary. Let's take an extreme case: free food. If I can eat for free 5 days a weekend, that's a lot of money I don't have to pay!
Comparing to the ingredient cost of making your own lunch (lets say $3), free lunch is only worth $750 a year. Maybe more if you eat expensive lunch items and you assign a time value because you hate making lunches that badly.
I'm in France, salaries are different. I'm around 40k/year.
In my case, I use about 150-200€ for midday lunches in a normal month (eating out, I'm in a client's office, and eating at my desk would means eating alone, and I forbid myself to do this anymore). So that's 200*12 every year.
If it's free food, it's money I don't get, so I don't have to pay taxes on this money. I'm in the 30% range, so free food = about 3k€/year salary.
I think the extreme perk there is "5 day weekend", not "free food".