Below is a list of showing you how much upgrades cost for each type of card.
Common Card
Upgrade to Level 2
2 Cards, 50 Coins
Upgrade to Level 3
5 Cards, 120 Coins
Upgrade to Level 4
12 Cards, 300 Coins
Upgrade to Level 5
25 Cards, 600 Coins
Rare Cards
Upgrade to Level 2
2 Cards, 80 Coins
Upgrade to Level 3
4 Cards, 160 Coins
Upgrade to Level 4
10 Cards, 400 Coins
Upgrade to Level 5
20 Cards, 800 Coins
Epic Cards
Upgrade to Level 2
1 Card, 100 Coins
Upgrade to Level 3
3 Cards, 200 Coins
Upgrade to Level 4
8 Cards, 500 Coins
Upgrade to Level 5
15 Cards, 1000 Coins
Legendary Cards
Upgrade to Level 2
1 Card, 120 Coins
Upgrade to Level 3
2 Cards, 250 Coins
Upgrade to Level 4
5 Cards, 600 Coins
Upgrade to Level 5
10 Cards, 1200 Coins