Here's a short parable based on a childhood experience.
I remember when my sister brought home a puppy and my parents . . . were not happy. We won them over eventually. We kept the dog, named him Winston and made all the usual promises about feeding, grooming, cleaning up afterwards. The whole drill. I remember Winston the puppy was great fun, sometimes you'd stay up all night with him, good exploring pal, active with basic issues like security and comfort and chasing things down, and, amazingly, the puppy was free. A puppy is plain fun for kids.
Nowadays I'm a parent myself and have learned a thing or two about hidden commitments. I am well-aware of what the fixed cost of buying something free is: it's nothing, right? What is not fixed is the cost of owning something free. And when you get into cost of ownership, cost of maintenance, cost of curation, cost of on-going conduct and procedure, you are way beyond the discussion of whether or not something is free.
[Side note: I think dogs have enormous value to all of us -- I'm right there with Naturalist Konrad Lorenz (Nobel prize for the geese-imprinting discovery and author of the important ON AGGRESSION) when he said "the rise of civilized man is indistiguishable from the rise of civilized dog". Dogs can be a major source of awesome. ]
Winston The Puppy cost nothing but Winston The Dog was expensive. All of the hidden "lifecycle" costs of Winston — and there were many -- hid in the years of maintenance, feeding, care, walking, medical and on-going ownership of Winston The Dog. In fact, Winston lived as an adult dog at my parents' house long after we kids had moved on with our lives. None of us even lived in the same state as my parents. Winston The Dog grew older, became geriatric, and in need of all sorts of attention that we had never imagined when he was a pup.
So, end of parable: that open source code living, eating and working in the heart of your IT infrastructure or processes -- are you honestly going to tell me owning that puppy is free?