[ApK]

Agree with Sleepybob.
If I understood the gist, the article seems to make the argument that from the PUBLISHER'S point of view, the mid-list is the mid-list and best sellers are best sellers, and any best seller will bring them the same money and any mid-list will bring the same money, so what specific books they put out for sale in those categories doesn't matter. And therefore it should be the same from the READER'S point of view.
I found that silly.

To DNSB, I think I was the first to bring up the word here (which I'm starting to regret).
I meant it just as you say: interchangeable. One dairy's gallon of 2% milk is very likely interchangeable with another dairy's gallon of 2% milk. I may have a preference for one, but if it's out of stock, or too expensive, I can buy another brand and my 2% milk-needing recipes will come out just fine. Get choco milk instead, then not so much.

It was in direct response to the comment that "if a book is priced too high, just buy a different book."

While a semantic discussion of the term "fungible" and it's meaning in economics and other contexts would be useful and interesting, it wasn't my intention to skew the thread that way. All should please read "simply interchangeable" where I wrote "fungible" if that helps.

ApK