I don't think we can know with 100% certainty. However, that doesn't mean that a correct answer doesn't exist.
This is why we can look back on our decisions and say "that was a good deal" or "I should've done something better" etc...
Ice cream flavor is a pretty trivial and inconsequential thing to analyze, which is why we don't care to do it. There are more consequential examples we could use, such as cigarettes or bioweapons. Is it actually worth giving up money for cigarettes, or for bioweapons? I'm sure it depends on context, but I do think there are right and wrong answers to that question.
I certainly think that the economy is better off when individuals come to these realizations by themselves, instead of having a central authority dictate them to the masses.
But still, that just means that decentralized economic systems are better than centralized economic systems. It doesn't mean that value itself is inherently subjective.