Idea is simple. It makes sense to me, but it may already be this way and I just haven't paid close enough attention, but...
Assign both a cap as well as XP value to each material component. Experience gain and base value are based around these values. Probably assign a small bonus to items which require a blueprint (10% or 15%?). What the value of each base component would be is a tough call, but can be figured out later.
Experience earned and base cap value from items become normalized, so that there isn't any one single item that outshines all others for leveling or making money. An item that has 1metal and 10gunpowder is just as
efficient for leveling/making money as an item with 1metal and 5gunpowder. If an item has its components changed, so would its item value.
BONUS SUGGESTION: Provide xp/value bonus if it is a quality/improved/superior/advanced/unique item.