The cost for a common booster pack is 1000 gold. It contains only 1 card, usually a common card, but there is a chance to recieve a card of greater value.
The cost for a ancient booster pack is 50 runes. You may also gain ancient booster packs from accomplishing levels or from battles in the campaign. It contains 2 cards, one of them is at least a uncommon card, but there is a chance to receive a card of greater value.
The cost for a legendary booster pack is 100 runes. It contains 4 cards, guaranteed at least one rare or epic card, but there is a chance to receive cards of lesser value.