Printer ink is now regarded as the most expensive liquid in the world -- ahead of Champagne, Chanel No. 5 and even human blood -- but there is no reason for it to be so expensive. Indeed, replacement cartridges cost around 23 cents to make but often retail for around $60. It's an economic trend called the "razor and blades model", where an item -- in this case a printer -- is sold at below cost, but the complementary consumable products, such as ink cartridges, are sold at a disproportionate price in order to recoup the money. Tech correspondent Helen Baxter explains.