Last times almost all summoners card prices went up but still there's one rare summoner is cheaper compered to all other summoners. So the cheapest rare summoner is fire. I myself prefer to play with fire team and most of the times when I choose Goblin Mech together with fire summoner l end up winning. I think that fire team will be more popular and more people will start playing with fire which will cause the price increase.
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From @zen21