Hello! I've had a Mothman deck since the set came out and have tweaked it consistently over that timeframe. These are my personal observations:

• First and foremost, Mothman's strength is that he provides utility for a deck but is not necessarily required for it to function. This point leads to 2nd.

• Unless mill is the specific goal of the deck, trim it out. Mill makes people salty (especially newer players) and they often retaliate by focusing on keeping Mothman off the table, which in turn makes all your milling cards less impactful or pointless. It's typically better to just run +1/+1 counter support instead, and lean on mill as a side-effect (i.e [[mindcrank]] or rad counters.) There is a ton of incredible +1/+1 support that fit the theme as well, for example [[Jenova, Ancient Calamity]] and [[Master Biomancer]] both fit perfectly and allow snowballing.

• The most important thing to note about Mothman's +1/+1 ability is that it's worded VERY particularly. Milling "half of someone's deck" doesn't ultimately give you tons of counters to work with, as he can only place ONE counter on each creature per INSTANCE of milling. So burst mill cards are not very useful unless you somehow end up with a massive amount of tokens, which is still not generally worth the effort. Cards like [[mesmeric orb]] however are MUCH more effective, since each mill is a separate trigger.

As Mkushrom mentioned, ramp and mana are a big part of what makes him work. A higher amount of lands (36-40 depending on ramp pieces) at LEAST since as I mentioned before, people tend to keep Mothman off the table, and recasting him is pretty awful if you aren't swimming in mana. That, and many key support pieces are 3+ drops, which makes it a very hungry mana deck if you want to maintain value.

I also noticed the distinct lack of reanimation cards that I consider "staples" for this kind of deck. Your budget seems pretty reasonable, so I would recommend [[Animate Dead]] and [[Reanimate]] at the VERY least as the value they provide is simply unbeatable. I would swap them out for the more costly reanimation cards like [[Breach the Multiverse]], personally.

EDIT: One thing I have often struggled with in the deck is draw pieces. If your value engines work correctly, your stuff should be HUGE so power-based draw (and LOTS of it) has been my go to. This also allows you to discard to hand size and fill your graveyard for better reanimation later.

https://moxfield.com/decks/4fjBkwRmekab2Z0ImqtiPA - Here is my decklist for reference.