The retail cost of printer ink itself costs more than most liquids on the planet, including oil, liquor, milk, and even space shuttle fuel. The HP20 black ink cartridge has only 28 ml of ink and costs an average of $40 per cartridge. In order to get a liter of ink, you would need to have around 36 ink cartridges. Therefore, the cost of a single liter of HP ink is $1,440.
So that gallon of milk is sounding fairly affordable now, huh?