Because the only way to do price finding is to make an offer and have customers accept it. [...] So selling with a low (or free) price does not give you any moral rights later to say it was too low, give me more.
You're thinking of this in a very narrow way.
If I tell you "here, have this for free, no strings attached", and then after you use it, I jump out of the bushes and say "hey! That thing had value! You're morally obligated to pay me!", of course you are under no such obligation. That seems to be the situation you're thinking of.
What if you open your imagination a bit? What if I say, "if I tried to maximize the amount of money I made, I would have to price out people who wouldn't be able to pay anyway and their lives would be better off being able to use what I built. I don't care about maximizing the amount of money I can make, I'd just like fair compensation, so instead of setting a specific price I just ask you to 'tip' a fair amount if you can."
Am I "morally entitled" to any specific amount of money, if I didn't set a specific price? Not really, no. But it's not like I'm saying "please, have this for free, I don't want anything back, seriously", instead I'm saying "I would like something back, but I'm trusting you to decide on a fair amount".
Here's the more interesting question: if you're not obligated to pay a specific amount, can such a "trust" system work? Can I get fair compensation, and still allow anyone who wants to be able to use my work?
As it stands today, in most cases, probably not. But open your imagination a little. Can it be made to work? Can there be public pressure on major companies to contribute to TideLift and/or OpenCollective? Maybe non-commercial source-available licenses, which still contribute to The Commons, even if they're not the traditional understanding of "open source"?