I worked at a used book store for years, and I'd say that $6 was the threshold for a decent book that there was some meaningful demand for, and ~$3 was a good median price for a regular shmegular paperback, with a little bit more or less depending on level of demand. Used books was an interesting business to be in, because you were checking against market demand every single day. If a book hadn't been selling, you marked it down and charged less for the next one. If you priced a book and it sold within minutes, you knew you should charge more for the next one. If publishers and retail bookstores can't be bothered to pay attention to what customers want and what they'll pay, then they deserve to not make sales.