First, you add any extra mana youre required to pay, then you subtract the discount from your cost reducers
The longer the game goes, the fewer cards the opponent will have in hand and if they end up without anything in hand, Dispersal will likely be doing a fairly handsome Vindicate impression one that can get around hexproof, at that
1 Generous Gift
Disfigure doesn't sound great, it sounds pretty ripped off when you think that blue got Mana Leak, painless draw and Unsummon which are more rounded abilities with higher CMC
Ritual of the Machine There arent too many clean ways to take control of a creature in mono-black, as youre usually relegated to getting something back from an opposing graveyard
Plus, you need to put your opponents' life totals to zero anyways, so it's a win-win situation