An important factor in the measurement of any performance marketing / direct response effort is not only the response rate (number of responses), but also the quality of response – how many paid up, how many returned the ordered product, how many of those receiving product on approval failed either to pay up or return the book, how many were successfully upsold etc.
A small response from a high-quality (well targeted) list is often better than a larger response from a poor-quality list.