Philosophers, artists, and other thinkers have written about aesthetics for thousands of years.
Truly answering your question would take a long time, but a short version might be something like: the collective opinions of a diverse group of highly informed and experienced (not necessarily educated as in a university) individuals. But that's just one approach and there are many others.
Even then, anything is probably superior to using monetary value as equivalent to anything other than what the market currently prices it as. I certainly don't think the most expensive painting is the most beautiful one, although it may be not be totally uncorrelated.
If you're interested in these kinds of questions, definitely look into the philosophical field of aesthetics.
Truly answering your question would take a long time, but a short version might be something like: the collective opinions of a diverse group of highly informed and experienced (not necessarily educated as in a university) individuals. But that's just one approach and there are many others.
Even then, anything is probably superior to using monetary value as equivalent to anything other than what the market currently prices it as. I certainly don't think the most expensive painting is the most beautiful one, although it may be not be totally uncorrelated.
If you're interested in these kinds of questions, definitely look into the philosophical field of aesthetics.