From The Independent:
A Society of Authors survey five years ago found that half of all authors made less than the minimum wage, and that three-quarters earned less than £20,000 a year.
"Most authors struggle," says Mark Le Fanu, general secretary of the body. "The gap between the few top authors and the rest of them is widening all the time," he says. "The vast majority of authors earn very little and most authors keep up their job until they can afford to write full time."
"If you are going into writing to make money, you are going to be disappointed," says Camilla Hornby of top literary agents Curtis Brown. "Most writers write because they are passionate about writing: it is easy to spot the ones who just 'write by numbers'."
Literary agent Anthony Lownie adds: "Unless you have a rich aunt somewhere who can support you, don't give up your job," he says.
Many well-known writers have held other jobs while writing. Poets Philip Larkin and T S Eliot held full-time jobs all their lives, as a librarian and a banker respectively, and writers throughout the ages have been journalists (Dickens), advertising executives (Peter Mayle, Fay Weldon and Salman Rushdie), teachers (Nick Hornby), stand-up comedians (Jenny Colgan) and even cab drivers and dentists.
[And even editors.]