If I name an unenchanted pickaxe, its one level, if I add a name to one while adding other enchantments, it adds one level (even if the pickaxe has other enchantments), but if I name an enchanted pickaxe without adding another enchantment on and just naming it, it costs however many levels it would cost to put an enchantment on at that point. For example, I had a pickaxe with mending, silk touch, and unbreaking 3 in that order and to add efficiency 5 and name it cost I think 17 levels but to just name it cost 16 levels.