5,000 units
Just to expand a bit on yesterday's post: When I first went to work in publishing, my employer kept copies of Publisher's Weekly in the break room. Publisher's Weekly is the industry magazine--emphasis on industry. Interested in writing and literature? Publisher's Weekly is not for you. Interested in how the ever-fluctuating price of paper is affecting profit margins? You've found your read.
Indeed, Publisher's Weekly displayed at times quite a bit of resentment toward writers, who were generally seen as a bunch of whiny prima donnas who would occasionally do horrible things like switch publishers to get a better contract. (Remember: It's OK for publishers to be focused on the bottom line--it's just business. But writers should be above that sort of thing.)
I remember in particular an opinion piece whining about writers who whine about publishers. The guilty writer in question was a poet. He was complaining that publishers don't promote writers enough, and his last book, the woman who wrote the piece pointed out, had sold a mere 5,000 copies. In what other industry, she asked, are you expected to bust your ass marketing a product that sells only 5,000 units?
At the time I was fresh enough out of college, where I had majored in English literature, to be shocked and somewhat angered by the books-are-just-widgets mentality. (I am paraphrasing the piece from memory, but yes, she did refer to poetry as a "product" that in this case sold 5,000 "units.") On the other hand, I had to admit that she had a point--spend too much money on something that inherently has a pretty limited appeal, and you will be out of business very soon. From a business perspective, why would anyone be interested in a book that sold only 5,000 copies?
Fast forward to today. If the poet had sold 5,000 copies of his book as a $3 e-book on Amazon, he'd make $10,000 off it--not a fortune, but definitely a nice addition to the income he gets from his grants and his teaching gig. (And I'm sure $10,000 is quite a lot more than the guy actually made from his book back in the early 1990s.) Amazon would make a mere $5,000, but Amazon is set up to chase the "long tail"--to cheaply market, say, ukulele books to ukulele enthusiasts and poetry books to poetry enthusiasts--so they can make money off books that don't sell tons of copies.
This I think shows how dramatically the publishing business is changing. Selling 5,000 units is no longer something to be scorned--people and certain corporations can make money doing it.