You can set the price of each item that you are selling according to the profit you wish to earn from it. If you don't add profit to your items, then all sizes will be made available at the production price.
The production price varies according to the paper type and size of your printed items.
If you are selling a custom item, all the logistic aspects of your item will be handled by you. Thus, the price you specify should include necessary shipping costs and handling fees.