If you want to see the "base price" without your mark up, check your profile. Do this without logging on and you your will see the prices with your markup added in. You also get a small percentage of the sale as profit added to what you put in the blank for each size.
When I go to "art prints" on my profile, I see that the first 4 sizes are all $12.26 before markup is added. You can add a progressively higher markup to each size all the way up to "expensive" for large prints on a progressive curve (5,7,10,15, 25,50, etc). If you feel generous, you can give the cards away cheap.