The curious absence of the online magazine rate card

Why the reluctance by magazines to publish online rate cards? While it wasn't exhaustive, a quick review of the websites of various consumer magazines shows that many don't post a rate card for buttons, banners and the like on their sites, with the kinds of frequency discounts and size premiums that are commonplace in print. If you enquire about such a grid, there is a curious reluctance to comply. Everything seems to be done on a case-by-case basis.(Some magazines even demur to post a print rate card, of course, requiring a form of registration or a conversation with a sales rep before they'll deign to send you an e-mail link to download it as a pdf.)

Even a company like Rogers that does publish an online card (see below) that applies to all its consumer titles, is curiously coy about audience and readership and the usual trappings of a print rate card.Is it because the whole online field is so fluid? Is it because they want to use online ads as a bargaining chip in negotiations with advertisers? Or is it because they don't know what to charge?