What the Forge charges to upgrade an item, enchant it to a higher rarity, or upgrade a Valorplate Shard. Prices go by the item's tier — the game's internal item scale, shown on each item's own page — not by your level.
What “tier” is, and what an upgrade actually changes
The Forge never shows you a tier. It shows the upgrade rank, 0 to 5. Tier is the item's own power rung, separate from your character level, and it is what the price tables below are keyed on. These are the file's numbers, not a reading of the screen.
An upgrade does not change the item's tier. The tier bands in the tables are a price lookup: an item sits in one band for its whole life and gets dearer to upgrade the higher that band is.
Each rank reads the Primary Attribute one tier higherDatamined. Rank 1 is tier + 1, up to rank 5 at tier + 5. Nothing else on the item moves with the rank on its own.
Rank is gated by rarity. ranks 1, 2 need Common or better; ranks 3, 4, 5 need Rare or better. So the rank you can reach depends on the item, not on your materials.
An improved (gold) trait is read 100 tiers higherDatamined, and in this game that means exactly double: of the 160 rows the game's item-scaling table holds per rarityitemScaling, rows 101–160 are rows 1–60 with every number doubled (8,700 scaling values measured on this build). Which traits an upgrade improves is decided server-side and is UNKNOWN; only the three secondary slots the table whitelists are eligible.
Masterwork is the trait you choose at rank 5Datamined. It is a slot of its own, not an improved version of a trait you already had — which is why a Masterwork line and an improved line are both drawn gold Datamined but mean different things.
A consequence worth stating: because the improved half only covers tiers 1–60, an item above tier 60 has no row for the game to read an improved trait from. The trait tables allow tiers up to 100, and the price bands below go that far too. Whether items between 61 and 100 exist is UNKNOWN: the tables disagree with each other and no test settles it.
Upgrading an item
Each upgrade is one rank, and each rank has its own price. Ranks 3 and above need a minimum rarity.