The subject of this question is the Forge Cleric's debatably useful Channel Divinity ability, "Artisan's Blessing"
You conduct an hour-long ritual that crafts a nonmagical item that must include some metal: a simple or martial weapon, a suit of armor, ten pieces of ammunition, a set of tools, or another metal object (see chapter 5, "Equipment," in the Player's Handbook for examples of these items). The creation is completed at the end of the hour, coalescing in an unoccupied space of your choice on a surface within 5 feet of you.
The thing you create can be something that is worth no more than 100 gp. As part of this ritual, you must lay out metal, which can include coins, with a value equal to the creation. The metal irretrievably coalesces and transforms into the creation at the ritual's end, magically forming even nonmetal parts of the creation
While useful for making sure your party always has the equivalent of a general good store around with you, whether or not it can be used over several attempts to craft better goods is the subject of other questions. Here, however, I'm concerned with what is the value of an item.
Suppose the following situation: I, a Forge Cleric, am currently wearing a suit of scale mail. I decide it's time to finally upgrade to heavy armor, doff my scale mail (which I previously purchased for 50g), and decide to make it into a suit of chain mail (worth 75gp). Thus, I obviously lay out some coinage...
... but how much? Can I use the scale mail's full value and only have to add 25gp to the total? More? Ask my DM?