Simply using too much ink
Your MP500 is similar to my MP780. I don't think you can get anything from the stores. It is hard to tell if the yield of 200 is accurate or not. It probably is but a lot depends o the images and text you are printing. If you print the color red, your yellow and magenta cartridges will be used to produce red. So in effect you will use up 2 colors to produce one color. To print something in greed your printer will mix the Cyan and Yellow (I think) and for blue it will mix Cyan and Magenta. So if you are wanting to print something for your own use, I suggest you print in black/greyscale only and in draft mode. However for presentation, I think it is best you do the standard printing. Black in draft mode I understand only uses the PG5 black cartridge. If you print anything in colors specially pictures, there is no way to accurately determine the yield and not a good idea to scrimp on the ink or else you get a poor quality picture.