Tested this on Tabletop Simulator today against some REALLY powerful decks... and won!
Couple of suggestions, and trying to keep within a $50 budget:
Lurching Rotbeast - Cycling isn't good enough value to keep this in - Swap with
Butcher GhoulMire Triton - This looks like the only Self Mill card in the deck, and you do not have a ton of Recursion... I like
Mire Triton, but I think a simple
Murder of
Go for the Throat if you have one would be a better fit
Sorin's Thirst - For a 2 mana instant, there are more impactful options. Look at
Pharika's Libation, 3 CMC, but adds versatility in Enchantment removal that normally doesn't exist in black
Liliana's Contract - Would be a great, sneaky alternate win-con... except you only have 3 Demons total. I don't have a direct replacement for this, but here are some other cheap card suggestions that seem to do what you want:
The Woe's =
Woebringer Demon to help deal with opponents creatures,
Woe Strider for a free sacrifice outlet, Woe Bearer for some recursion