When you want to do an upgrade and do not have enough copies of a card to complete the process you can use Fusion Stones to craft your missing card(s). The cost in Fusion Stones depends on the rarity of the card. The list below is a guide.
Common Card
Cost: 1 Fusion Stone
Rare Card
Cost: 5 Fusion Stones
Epic Card
Cost: 20 Fusion Stones
Legendary Card
Cost: 50 Fusion Stones