Liz, I told you above why it is. If you want me to take it further... my own feeling is that to be competitive printers are sold for at best no more than the development/manufacturing costs. ie. when you bought your Lexmark, they would have made no money on that (although the retailer would have).

From there on, it's a gamble but they will try to persuade you that the only safe ink and the only ink that will not invalidate warranty is thier own. If you do follow that, it is quite clear that they are soon quids in!