Board Thread:Question and Answer/@comment-115.66.228.161-20150420133022/@comment-25151790-20150420205052

Upgrading a non-maxed card is always (current_level*10 + 30) gold per card used.

The gold per card to upgrade a maxed card varies depending on the rarity:
 * SR = 1060 gold
 * HSR = 1260 gold
 * GSR/UR = 1460 gold
 * HUR = 1660 gold
 * GUR = 1860 gold