You are confused because dots on an image do not correspond to dots on a printer.

The recommended 300 DPI is for images where each dot is actually a pixel and can be of any number of colors, 16 millions for a JPEG, more for other formats.

A printer requires many dots to render a single image pixel, sometimes over 100. That is because a printer has between 3 to 12 colors possible for each dot. In order to simulate one of these millions of colors, printers make small patterns (called dithering) to make your eye see one of those colors.

That is why so see printers specified with resolution of 2400 to 9600 DPI but images in the 72 to 600 DPI range. You cannot calculate the volume of ink without knowing how the printer renders each dot in your image. That is because light colors require fewer dots than dark colors since the white of the paper provides the white component.

Some printers will apply a gloss coating over based on how much ink was used for each pixel in order for there not to be variations in glossiness on the print surface. Some printers will even mix differently to compensate for some cartridges being low.