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Its hard to predict what will happen but if you are a player then it won't affect you as you wont sell cards. I hardly sell any card becase I am growing my collection and it makes my game better..

At what level is sustainable earnings expected in SPL battlings? What's the ROI on deck building ?

TBH,, I never did deep calculation but even now my investment is on profit if I sum whatever I hold in the game including cards and other assets.. Its like you keep on building and grow the account. You can cash out no regular income ..