If I use the "reduce the play costs of all of your opponent's Digimon by 1" effect and all of my opponent's Digimon have their play costs reduced by 3, does an opponent's Digimon with a play cost of 2 now have a play cost of -1?
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Official Ruling
No, it doesn't. The minimum value is 0, the resulting play cost can't be negative.
Source: BT16-060 Tankdramon