Just found this on Lifehacker, which in turn found it from the NY Times.

The gist of it is that printing at home costs around 28 cents per print whereas there are services that can do it for as little as 13 cents. This is due to the expense of photo paper and ink for home consumers.

Here’s the real standout from the article:

Printers return relatively low profit margins. But the ink, ounce for ounce, is four times the cost of Krug Clos du Mesnil Champagne, which sells for around $425 a bottle. Ink is about the same price as Joy perfume, considered to be one of the more pricey fragrances, at $158 for a 2.5-ounce bottle.