Remember too - it's not just the markup on the print you have to account for. You also have to account for what you went through to get the image, how difficult is it to duplicate, what kind of post processing did you have to do on it and how long did that take you, your investment in equipment, time, and training, how unique is it, just to name a few things. I'll be the first to admit that pricing is more art than science, but you don't want to get labeled as 'cheap' right from the get-go. Take pride in what you create.
Oh, and getting the word out about your prints (marketing) is the bulk of the battle here. It's rare that someone stumbles into your work falls in love with it, and just has to buy it on the spot.