Extortion Deck (Legacy)

Note that these suggestions will involve a hefty increase in price, so they will be somewhat costly to implement:

1 - Switch Evolving Wilds for Marsh Flats.  The Wilds fetch the land in a tapped state, while the Flats brings it into play untapped and ready to use that turn.  It doesn't seem like much, but in Legacy that makes a HUGE difference.
2 - Switch Orzhov Guildgate for any of Isolated Chapel, Fetid Heath, or Tainted Field.  Again, playing a land tapped can make a HUGE difference.  With the Chapel, as long as you have a basic land it is a simple dual land; the Heath can be used for mana fixing if needed, or for colorless if the fixing isn't.  The Fields are the worst choice, but still a viable one.  Of course, Scrubland is the best black-white dual land, and can even be fetched by Marsh Flats/Evolving Wilds, but are much more expensive than the other options listed.  Also, you can use more than one of those options, just switch out basic lands in place...but make sure there's a few basics for the fetch lands to get.

Beyond that, I'm not really noticing what the primary goal of the deck is.  You have a ton of cards with only one copy, which makes consistency a tough thing to accomplish, so I might be able to help a bit more with the rest of the deck if I knew what cards were integral to it and which are expendable.  Once that's done, we'll work out your best options and tweak the mana pool a bit more (once there's a black-white ratio worked out).