The smaller the school, the more it's going to cost to print. Or if it's a big school, they may have opted for the larger sized (bigger pages) yearbook that costs more. My high school used a smaller sized book, and it was about a $4000 base fee for the book (more pages = higher base fee) + about $40 a book to print. More copies would have meant a lower per-copy price but the same base fee. Of course the base fee is spread across all of the books. So if you figure 100 books using those figures, you're looking at $80 a book. If you had 1000 copies and the printing cost dropped to $20 a book, add in the base fee and it's only $24 a copy. I'm not sure how much the printing cost drops as the number of copies go up.
Many schools subsidize the cost of the book, so even if you're only paying $15, it may really be costing you more than that when you figure in the amount the school paid for each copy. Of course with a public school that's divided among the taxpayers, not the families of the students as it was in my private school.