“People are generally surprised that we make a living from it,” Mr. Munroe said. Without being specific, he said that the sales of xkcd merchandise support the two of them “reasonably well.” He said they sell thousands of T-shirts a month, either of panels from his strip or in their style, as well as posters.
http://www.nytimes.com/2008/05/26/business/media/26link.html
“People are generally surprised that we make a living from it,” Mr. Munroe said. Without being specific, he said that the sales of xkcd merchandise support the two of them “reasonably well.” He said they sell thousands of T-shirts a month, either of panels from his strip or in their style, as well as posters.