The major costs you're missing are marketing and "plate" (up front cost to produce the content). Those make up most of the total costs. For textbooks, the decision makers are professors (so door-to-door sales to get their attention), and the market is pretty small, especially for upper level content (so few units to amortize fixed costs over). Print, paper, and binding are cheap, say ~$10-12 average for a textbook. Typically, distribution channel takes a 20-25%, depending on channel partner, and many colleges mandate that sales go through the school bookstore because they get a cut, so publisher's website isn't necessarily a viable option (without a lot of student marketing). Author royalties run ~13-15% of revenue, and editors hit plate expense (they're publisher employees, so not a variable cost like authors). Textbook publisher Ebitda margins wind up running 20-25%, but most publisher's pay a lot of interest expense, partly because the major costs are up front, and partly because there's been a lot of PE ownership. Net margins can be tight as anyone else's.

Source: worked for a plaintiff publisher in this case. Think Pearson, Cengage, and MHE all publish financials also.