Most Epson printers use separate printer ink cartridges which one cartridge for each colour ink. Epson S21 ink cartridges are consists of four individual printer cartridges because the printer has a four-colour ink system. Genuine Epson ink cartridges use DURABrite ink, which is a technology developed by Epson to help protect prints the second they leave the printer. DURABrite also help prevents prints from fading over long periods of time cause by exposure to UV light. Epson ink cartridges (and pretty every other brand of ink cartridges) have a reputation of being very expensive and S21 ink cartridges seem to follow in these footsteps, whoever they are no more expensive then any other ink cartridges for Epson printers. If you find the genuine Epson inks to be too expensive for you then there are cheaper alternatives avaible in the form of compatible ink cartridges from third party manufacturers. These compatible in cartridges can cost up to 60% cheaper than the original Epson ink cartridges and they contain the same amount of ink so print the same number of pages with quality that difficult to tell part from the genuine inks. Compatible inks for the S21 are available from all good retailers of inkjet cartridges and come with money back guarantees.