How many books have to sell to make a profit?

An item from Digital Book World I cannot resist reposting, as I find it so pertinent and amusing.

Dunham's $3.7M Payday and What It Means


Even when self-published works can climb to the top of best-sellers lists, we're still in the era of the big book. Despite there being more manuscripts up for grabs than ever (at discount prices), publishers are still making big bets for certain titles.

Random House paid a reported $3.7 million for the U.S. and Canada rights to Lena Dunham's upcoming book, Not That Kind of Girl: A Young Woman Tells You What She's Learned. At, 26, Dunham is a relatively new member of the New York glitterati but already has several impressive credits to her name, including the feature film Tiny Furniture and the HBO series Girls, both of which she created and stars in.
How many copies of the confessional will RH have to sell to make a healthy profit? The math is complicated but the simple answer is "a lot."
How many copies of On Dublin Street, the self-published hit, does Samantha Young have to sell to make a profit? Well, it's not a completely fair comparison, but you get the idea.