Would the summoners be counted as starter cards? if so why not implement a spellbook level system, letting you pay for a higher lvl spellbook containing starter cards matching the level of the spellbook? That actually sounds useful maybe 0-5 with each lvl costing 10 bucks, maybe even implement a way for old players to burn dec to lvl their spellbooks. With a zero level spellbook letting you play but not giving any starter cards, so youd have to build your own deck.