Why is there a price difference between products with the same weight?

The price difference between two products of the same weight come from their margin level. The margin (also called "premium") is the part of the price covering our costs and reseller's margin, it varies depending on the product’s rarity, craftsmanship, and its associated costs (production, handling, warehousing, etc.).

Therefore a cast bar and a Lady Fortuna minted bar of the same weight will have a price difference due to the Lady Fortuna's higher production and craftsmanship costs.