I think NPR covered this pretty recently. So, first you have a book salesman that comes in and sells the modifications from the previous versions--that its a better educational tool. They don't talk a lot about price, and they make the sale. Then, you have a super limited market. You aren't going to print a lot of these books because there are a limited number of students who need the book. After the first year the sales plummet because used books exist. So, there's a short window to make back all the money on the book and also profit enough for it to be a legitimate business. Then, there are people who hoard books like this playing on seasonal demand--buy low, sell high. There are programs they can buy to help them maximize their profit. There are even people who scour used book stores for good finds with barcode scanners.

And, I'm not sure where common curriculum comes into this, or if it does.