Assuming there's some budget constraints here, as the deck looks quite casual. With that in mind, I do have a few suggestions. First just the major deck building issues.
1) Your curve is waaaay too high. Even on the kitchen table, I rarely expect to cast cmc 5+ cards. It will give you 11 more slots to work with. Cutting your four drops increases this number to 19.
2) Too many 2-ofs. If you want a card in a focused deck like mill, you generally want 3-4 copies of it.
3) You have no way to survive incoming
aggression, aside from the two copies of
Fog Bank.
4) Your mana source distribution isn't bad, but there are better budget lands than
Dimir Aqueduct.
Choked Estuary,
Dismal Backwater,
Sunken Hollow,
Temple of Deceit,
Fetid Pools,
Field of Ruin and
Underground River are all under $4 a copy.
Drowned Catacomb will go down when it rotates from standard next year.
As for actual budget cards to look, there is quite a few solid options:
1)
Jace's Phantasm will win a good chunk of games on its own as a 5/5 flyer. It is a great blocker and threat at one mana.
2)
Manic scribe is kinda like a poor man's
Hedron Crab. It mills three when it comes it, blocks, and can get some extra value over the course of a game.
3)
Breaking // Entering is the closest thing you have to additional copies of
Glimpse the Unthinkable. Milling 8 for two is pretty solid.
4)
Crypt Incursion will help you survive against creature based decks, and it a bit of gy hate main for dredge decks.
5)
Fraying Sanity is a bit worse without
Mesmeric Orb or other mill to use on their turn, but is a good value card.
6)
Bontu's Last Reckoning/
Languish are pretty much your only budget options for a black sweeper in modern. I wouldn't run more than two maindeck, and would honestly go for the reckoning just to get rid of larger critters.
7)
Hedron Crab, while a lot worse without fetchlands, still gets some good value. It's a little higher in price ($5-6) but still really good in the deck.
In the $5-10 area is
Visions of Beyond,
Shelldock Isle,
Fatal Push, and
Archive Trap.