Bci-21/bci-24, both are bad. Thier is an easy way you can compare ink usage with canons. go to canons official site and see how many pages the cartriges are rated for. Then compare retail prices for the cartriges to get a cost per page. Also keep in mind that these ratings are assuming that you use up all the ink. If you have a color cartrige that has all three colors in on cartrige, also keep in mind that when one color runs out, you throw away the rest. Some of the cheaper canons actually perform very well, but if you pay a little more for a canon that uses bci-3 or bci-6 cartriges, you will save big time in the long run. Also, bci-3 and bci-6 cartriges have a decent number of compatible cartriges avalable, and if you wnat to refill, thier are empty replacment cartriges for both that are specifically designed to be refilled. With the right ink, it is every bit as good as canon ink for 1/10th the price.
Hp, lexmark, canon, epson all play the ink game. They offer good quality printers for far below cost, or even free, then they soak you on ink costs and they know the undersized all colors in one cartriges are very hard to refill (for canon anyway). If you can in any way affor a printer that uses these cartriges, I strongly recomend it. Check the canon sight and compare cartriges for yourself.