The pricing for print sizes, individual image instant downloads, category instant downloads, and single-image products is set within your pricing sheets. Pricing sheets can be created and modified at the following location: Selling » Pricing, Shipping, Tax » Pricing Sheets
Pricing sheets are associated with your galleries and the images within them. Changing your print prices within an existing pricing sheet will result in the print prices being changed for all galleries and images previously associated with the modified pricing sheet.