Both laser and inkjet printers are notorious for requiring expensive replacement cartridges long before you really need to. It's actually epidemic in the industry, to the point where class action lawsuits have been filed against Epson and Hewlett-Packard.
Enter Slate Magazine writer Farhad Manjoo. When his printer abruptly stopped printing, he began to wonder if the printer wasn't simply lying that it was out of toner in order to trick him into buying more. The prints hadn't been fading at all, but the printer simply refused to go on.
But with a bit of research, Manjoo saved his 60 dollars and instead covered up a sensor on the side of the toner cartridge with a piece of electrical tape. This tricked the printer into thinking the cartridge was full -- and the printer's been going strong ever since, eight months and hundreds of pages down the road.
If you want to find out what you can do to convince your printer to continue printing, FixYourOwnPrinter is a good place to start.