Smugmug's printing partners sell paper and ink. That's only worth a few pennies to a few dollars depending on print size and medium. Their machines don't know or care what image will end up on the paper. You are selling time, labor, vision, and skill. That's worth more than paper and ink.
I'm mostly a landscape/cityscape shooter, and there's a certain amount of effort that goes into each shot. This may be different that what someone charges for events, etc.
I am continually on the lookout for nice scenery and new points of view. I am constantly watching the light and clouds, and making mental notes of the times. Once I have something in mind, I may scout the location and pay more particular attention to the time of day I want to shoot. I keep in mind things I have learned from previous shoots, and I hopefully apply that knowledge in a way that will make this one better. I show up early, set up my gear, wait for just the right time, and take my shots. I may repeat this the following day if there is a chance that I can get something a little better. After I sort through my images and pick the one(s) I want to print, I spend some time in my conversion/editing software making things just right. Once everything is just right for me on my computer, I have to get it out in public view where it can be purchased by a potential customer.
Depending on circumstances, I have just spent some number of hours getting this shot from a picture in my mind to a picture on paper. It is now art + experience + labor, not just paper & ink. I'll charge what I feel it is worth just as if it were a painting or a sculpture.