The thing is that a lot of the costs are fixed, up-front costs. Editorial, typesetting, proofreading, copyediting, other production costs (fees, permissions, etc. for art), sales and marketing costs (catalogs, advance copies, sales calls) are all incurred before copy 1 is printed. Marginal costs for a book that sells well is relatively small, though some expenses (retail coop payments, royalties) get a bigger budget when a book is a bestseller. A big-selling book is way more profitable than a midlist book, which might not even break even. Traditionally, backlist (where those fixed costs were amortized long ago) was also profitable for publishers, even at lower sales volumes, but it is not quite as good business these days. It's really true that bestsellers are like the hollywood summer 'tentpole' movies--they pay for everything else.