I have no insights into the per-user cost for Meta to run their platform, but I have a hard time imagining that's what this is based on. I think it's much more likely that users are paying a price based on "what Meta could be making if they sold their data".

Pricing isn't based on operating costs but market value of services offered. That's how every business in the world works.

If Meta found a way to double their ad revenue from non-paying users, would you expect the price for this subscription to double?

Yes it'll actually have to more than double, since the high-value users (from perspective of advertisers) will more likely opt-in to the ad-free plan. The opportunity cost will be insane otherwise.

Or you know, keep doing what I have done for the last fifteen years: Ublock Origin.

Starbucks doesn't say "hey here's a free coffee", hand me the coffee, and then say I implicitly agreed to a 500 page lawyer-exploitable EULA by taking the coffee... and then later say "well, if you're enjoying that coffee you can give us $5 in cash instead.

Those are not equivalent and I wish people would quit pretending that they are equivalent. People "expect" things on the internet to be free because Meta and its ilk are saying they're free and then doing a myriad of shady things on the backend. Not because users are naive and entitled. Saying so does a disservice to the way the entire interaction takes place and the asymmetry that exists.

Money exchanged for goods is a very straightforward and honest thing. This is not.