Google and TikTok may seem free, but every click and scroll you make gets packaged into a "user profile" and sold to advertisers. Take Facebook, for example—it makes about $50 per user per year just from their data. In the internet age, if you're not paying for the product, you are the product.
In the context of the phrase " the free things are the most expensive" I would have thought you will be talking about, water, air, oxygen and the rest of the free things God created for mankind. However in addressing the free things you listed, they are basically not free, these are people's businesses you mentored, for you to use their facility, you have to pay for it, those apps were not invented from the beginning to be use on free bases, for the inventor to build it and put it on the app store, he definitely paid for it to be there, so you the end user too will have to pay for it.
I had the same perception too when I saw the subject of the thread, I was thinking that what the OP meant by 'free' was things that we don't pay for like air, rain water and so on. They are free but patients on life support pays for it and water can cost a fortune on a dry desert. Clearly what he meant was social media, that what we view for free are actually expensive but I think that is not true. Firstly social media platforms are people's businesses and they're in business to give services and make profit. Besides I don't think that social media is expensive, data subscriptions is what gives you access and it doesn't cost a fortune. Because data is not expensive compared to physical places where the rich and poor can go and socialize that is why it's very popular and many people visits it's different platforms everyday.