Envy

Bestow {6}{c} (If you cast this card for its bestow cost,
it's an Aura spell with enchant player. It becomes a creature again if it's not attached to a player.)
Whenever enchanted player taps a permanent for
mana, if that player has cast a spell this turn, that permanent produces {c} instead of any other type
and amount.
Each spell enchanted player casts during their
turn costs {c} more to cast for each spell they've
cast before during this turn.