It is value and empowerment, it is a support card not a payoff really and so it needs to be cheaper than four mana
Until end of turn, you may cast that card, and you may spend mana as though it were mana of any color to cast that spell. See, thats cool as hell
Flameblade Angel: great effect that can ping your opponent to death, but its a 6 drop rare that doesn't kill you in a few terms and that means its a guaranteed junk rare in Modern, and only somewhat playable in Standard constructed
So the only option left is the standard format where Dark Confidant is illegal, but even then Sorin shouldn't go any higher than $7-10 a card
It has a fantastic commander and a ton of useful creatures, all making the best use of Zinnia's (the commander) ability
Golgari leverages graveyard synergies to disrupt opponents and ensure you dont run out of resources