Level Up

Leveling up a card increases attack and health by 15% of the card's base values. The cost of leveling up depends on rarity and current level. All cards start at level 1.

Gold cost
Gold cost increases by 15% for each level (exponential). The cost is proportional to rarity of the card: Gold Cost = Rarity * 2300 * 1.15 ^ (Level - 1)
 * Common: 1x          =15%
 * Rare: 2x                =30%
 * Epic: 3x                 =45%
 * Legendary: 4x       =60%
 * Mythic: 5x              =75%

for leveling up to Level.

Normal Cards
Leveling up a card also costs a certain amount of items. Items come in tiers ranging from one star to five stars.
 * Elixir (depends on element). Earth Elixir for earth cards / Water Elixir for water cards / Fire Elixir for fire cards / Dark Elixir for dark cards / Light Elixir for light cards.
 * Necklace (depends on rarity). Common Necklace for most girl cards / Rare Necklace lace (i.e. Dark Queen, Dark Angel, Arabian Princess, Leprechaun, Snow Queen) / Epic Necklace (i.e. Vampire, Elemental Hunter) / Legendary Necklace (i.e. Undead Queen).
 * Misc (e.g. Wisdom Book / Precious Ring / Fairy Flowers / Nature Crown / Luxurious Diadem)

Tier 1 is Level 1 thru Level 5. Tier 2 is Level 6 thru Level 10.

Tier 3 is Level 11 thru Level 15. Tier 4 is Level 16 thru Level 20.

Tier 5 is Level 21 thru Level 25.

The following table shows the numbers for the elixir and for the misc item for common cards. For every rarity higher you need 1 elixir more and 1 misc item less.