In cases like this I usually make guesses at the value of [X]. With practice those guesses become more accurate. In this case I would assume [X] = 1 for now.
So if [X] =1, the increase of 20% to 30% is worth 0.5.
If I need to make the upgrade worthwhile, a 0.5 increase in value is not enough. So I should keep the Green Sorcerer at 7|4 at current cost, right ?
And, do you think this card has potential ?
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Edit : The unupped version now costs one more