Do you use cartridges in a laser printer?
Well they are called toner cartridges, but I know what you mean.
With laser printers you basically buy a cartridge full of toner powder, these vary in price from expensive for the original manufacturers ones throught to OEM and cheap refilled units. If you want the cheapest option you can usually just buy the toner powder and refill it yourself (some are more difficult than others, some need a chip to be replaced as well).
I have two different ones, one is black only, I can buy replacements for about £25 or refill it three times for about £15.
The other is a colour but the cartridges are very small, then again they only cost £12 each, but lots of black soon empties them.