Slate recently published a great article Take That, Stupid Printer! How to fight back against the lying, infuriating, evil ink-and-toner cabal by Farhad Manjoo. The title is a bit meaner than the article, which makes for interesting reading.
If you own a Brother HL-2040 printer, the article is especially relevant. The author suspected that the printer was lying about being out of toner and he figured out how to lie back to it, making it think there was a new cartridge. Sure enough the printer had lots of toner left, as Mr. Manjoo puts it "At least eight months have passed. I've printed hundreds of pages since, and the text still hasn't begun to fade."
Brother is not the only company wringing profits out of way-too-early warnings out being out of ink/toner. The good news, according to the article is that "... instructions for fooling different laser printers into thinking you've installed a new cartridge are easy to come by ... If you're at all skilled at searching the Web, you can probably find out how to do it .... Just Google some combination of your printer's model number and the words toner, override, cheap, and perhaps lying bastards."
My HP LaserJet 1320 is well-mannered; it warns when it thinks the toner is running low, but doesn't do anything other than warn. And, it's reasonably accurate, giving me time to order a new cartridge before it really runs out of toner. Apparently, I'm lucky.
Or, it may be that the more expensive the printer is up-front, the less the manufacturer feels the need to play tricks with the ink/toner. If that appeals to you, see Kodak's consumer printers aim to chop ink costs.