For example, if creating a Circlet of Persuasion, but just making it out of leather, it would cost the same as making it out of the described materials ("This delicately engraved silver headband...").
How does this apply when making it out of a more expensive material like gold (but buying the circlet first, then enchanting it later)? Would the cost of the circlet be subtracted from the crafting cost of the Circlet of Persuasion, and I only pay the difference?Another example is Boots of Speed. If I pay 150gp having boots made out of dragon hide, do the enchanted boots cost 150gp less to make?
Enchanted items (with the exception of armor, shields, and weapons) do not take the actual material into account in the price. A ring of protection costs the same if made from cheap copper as it does from solid platinum.
Magic supplies for items are always half of the base price in gp. For many items, the market price equals the base price. Armor, shields, weapons, and items with value independent of their magically enhanced properties add their item cost to the market price. The item cost does not influence the base price (which determines the cost of magic supplies), but it does increase the final market price.
In my experience, that is usually ignored. But by the rules, the cost given is for the enchantment. The base item should add to that cost.