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The problem with that option is that extra mana is always an advantage; and since DEC can be bought in the marketplace, you're giving more scope for people to win by throwing money at the game.

I was thinking more of pay X DEC from your won DEC from that specific battle.

For example: Sacrifice 50% of your potential won DEC for the extra 3 MANA. Lost the match, no DEC needs to be paid.