However, when I wrote my previous post I was more thinking about products in niche markets. An academic text by an expert in a narrow field would presumably appeal to fewer people than a Harry Potter novel, but to those who are working in the field, it might be very valuable to see the insights of an authority on the subject. Thus it becomes viable to publish texts that cost far more than a novel for each copy, if there are enough people still willing to buy it at that price. There are parallels in other niche markets, say a relatively obscure music style where fans are still willing to pay for recordings by a band they enjoy, or specialist software that is only useful to a relatively small number of businesses but is so useful to those businesses that it can sell for thousands of dollars per user.
Are those specialist texts greater works than Harry Potter because they sell for more? Are those specialist software products greater works than the latest Grand Theft Auto? I don't think that's necessarily the case. To me, a popular work that brings happiness to many people can be great, even if the highbrow brigade would look down on it and say it wasn't very good. When they've brought as many smiles to as many children as JK Rowling, their opinion on that subject will mean something to me. :-)
I want to close by coming back to the idea that value is often subjective. I am certainly not implying that to any given person a work can only be great if it makes a lot of money, just as in your own example of two people being in love, only two people's opinions on the greatness of the relationship really matter. I am just saying that in the large, as an economic matter, judging a product by how much others consider it worth is one of the few relatively objective measures we have available.
It's good to see different world views expressed clearly, but yours is incredibly foreign to me. For me, the number of dollars that someone else with much more money than I have is willing to pay for an object has almost nothing to with its greatness as art. I instinctively react against the idea as if you were saying "How can love between two people be considered great unless one party is paying the other a large sum to stay in the relationship?"
When 'brownbat' says commercial art, I presume he means art that is created because there is known demand from those with the money to pay for it. And when you say "unless you want to make some argument that art that is neither widely appreciated nor strongly appreciated by a few can still be great", this seems like it is totally ignoring the appreciation those without the ability to pay.
Why should we allow greatness of art be defined only by those who have accumulated a surplus of money? Sure, greatness is subjective, but when it comes to art, does it possibly make sense to restrict greatness to the subset of art appreciated by the rich? I would agree that art cannot be great if it is not "strongly appreciated" by at least someone, but can't it be subjectively great for that someone even if they can't afford to pay much for it?
The difference in our world view might be that I see "appreciative but unable to pay" as the default state of the majority of the world, and thus wouldn't think of restricting great art to that which is also financially lucrative, nor quantifying it by summing the product of the audience member's degree of appreciation times ability to pay.