Minecraft wiki entry for Best_enchantments_guide, associated text: There are multiple enchantment orders with the same cost in levels. At the same total costs in levels the cost in XP can still be different. For example two steps costing 2 and 4 levels require more XP than two steps costing 3 levels each. The optimal enchantment orders is {[(Helmet + Respiration III)@6 + (Protection IV + Aqua Affinity)@2]@8 + (Unbreaking III + Mending)@2}@9 with a cost of 6 + 2 + 2 + 8 + 9 = 27 levels.