[Taqwus]

Forge costs, if memory serves, are based entirely on the path/level requirements.
Level 1 = 5 gems, each level thereafter doubles (thus an item which requires level 5 in some path needs 80 units of that magical resource. A level 3/3 two-path item would require 20 units of each of the two corresponding resources, etc.)
Certain units (Master Smiths, and the new Ulm Iron Faith priest/mages) get innate forge bonuses (25% generally), certain items give forge bonuses (25% for dwarven hammer, 50% ea. for Hammer of the Forge Lord, Hammer of the Cyclops IIRC), spells might as well (never tried Forge of the Ancients, but description suggests it would help.).