If you're looking to play mill, this should give you a rough idea of where to start. I'm not going to guarantee a ̶h̶i̶g̶h̶ decent win rate, but this deck should be fun to pilot for an intro to the playstyle. As of September 2018, this list should cost you ~$100 on TCGPlayer.
Below, I will give my reasoning for each of my card choices.
Mainboard:
Creatures
4x Hedron Crab - AKA the most well known creature in mill. The crab is only as strong as the mana base, and in this case, we're playing mono U with lots of land destruction. Usually, fetch lands trigger the crab, but Field of Ruin will have to do, or you can target your own lands with Ghost Quarter if you're desperate.
4x Jace's Phantasm - Not usually in the high end version of the deck, but it will do work in the budget version. It can be used as a chump blocker, or it could even be an alternate win condition if it's a 5/5 flyer.
Instants
4x Archive Trap - The biggest and usually best mill spell. A lot of mana bases are greedy and there's lots of fetching, so it'll be likely free to cast. Works great with Field of Ruin, and sometimes is the perfect trap if your opponent searches after a Ghost Quarter.
4x Opt - Simple. Scry 1, draw 1. Card draw is great with Sphinx's Tutelage.
3x Supreme Will - Not usually in mill decks, but in mono U it can help you filter through your deck, or counter one of your opponent's threats.
4x Thought Scour - Also simple. Mill 2, draw 1. Some people will eventually upgrade this into Visions of Beyond.
2x Vapor Snag - We have no real removal in mono U. Good luck.
Sorceries
4x Mind Sculpt - This is as close to Glimpse the Unthinkable you can get without splashing black.
4x Tome Scour - It is the cheapest, most effective mill spell, but it is usually cut for something more powerful and nonbudget friendly.
1x Traumatize - Not as effective as it seems, but in mono U you might need to hit that bomb a$$ combo with Fraying Sanity to win the game. This will be one of the first few things to go as you upgrade, and you'll understand why.
Enchantments
3x Fraying Sanity - The newest mill staple thanks to HOD has given the deck a breath of fresh air. Still unsure of the proper number, but I'm going with 3 because it'll help speed up the process and it can combo with Traumatize.
2x Sphinx's Tutelage - It might not be the best, but at least it's consistent. With such a small budget, this or Drowned Secrets is as close to Mesmeric Orb that we can get.
Lands
13x Island
4x Field of Ruin - It’s forced search ability works well with Archive Trap and it gets you another land.
2x Ghost Quarter - See above, but it’s a may search ability. Or you can blow up one of your own lands to help trigger Hedron Crab again.
2x Ipnu Rivulet - It’s mostly here for its extra milling ability if needed.
Sideboard:
2x Engulf the Shore - Bring this in against Dredge and creature heavy decks. Still not real removal though.
4x Fog Bank - These might buy you a few extra turns, but it's going to be hard to decide what to take out depending on the matchup. You'll likely swap out Jace's Phantasm.
2x Gigadrowse - These are decent against a variety of decks. Being able to tap any permanent can be great. Playing against control, tap their mana. Big dudes, tap them too. You get the idea.
3x Negate - Just some more counters.
2x Set Adrift - GET OUT OF HERE LEYLINE! Or, honestly, anything you feel like bringing it in for.
2x Tormod's Crypt - Graveyard hate will come in handy at some point.
Upgrade Path:
If you're enjoying the deck, add about another $100 and you'll be able to upgrade. There's a few different choices you can make at this time.
If you wish to stay mono U, you can either invest in a playset of Mesmeric Orb or 3-4x Surgical Extraction. As long as these don't spike, you should have a little bit of money left over for whatever you please. Sphinx's Tutelage and Traumatize should be swapped out, along with another card of your choice.
The other option is to head over to (Budget 2) UB Mill...
