I sell just the images and charge 10.00 for basic ones and 12.00 for collage themes. The bakeries around here in Upstate NY charge $10.00. Some of the grocery stores charge you exactly what they'd charge for the cake to do it...so that is sometimes $14.00. I think when I figured it out, the frosting sheet costs about 1.16 a piece (depending where you bought them), and then I guestimate for the ink, and I figure each sheet is about $1.80ish. Again, that doesn't account for the ziploc bag I put them in or my time to make it and then take it to the post office for shipping. (I have an online business). If it is a friend of mine, I just charge for supplies....about $2.50.
I am annoyed with myself though. I bought the business card sheets and now that I have them, I think they are a waste of money. I wish I could do something with them...I'll need to get creative with them and put them on frosted brownies or something that you'd cut in that sort of shape.
I sell the most of the 2 inch round circles for cookies and cupcakes and the 1/4 sheets. One idea people love is if they are doing a full sheet cake, they will do the 1/4 sheet image in the middle and then stick the 2 inch circles randomly around the top of the cake. I normally do all different photos of the person, so it looks really nice. I say I charge more to do the different photos on the circles, but I never do.
Hope this info helps!