I agree with randym77. If there is a rule of thumb it would be it depends on the demand for your prints. I believe Rosity's print shop failed because they marketed to the wrong crowd. It doesn't take a marketing guru to tell you selling art prints to other [starving] artists is not the way to go. You need to find who's looking for art such as yours and market THERE. While Cafepress and Zazzle can get the prints done, you'll still need to work to find the customers. On an upbeat note (kind of), popular artists weren't always popular. Study those that do similar work and see what they did.