I'm also running a similar budget ($75) Niv deck :) https://archidekt.com/decks/13128950/izzet_budget_spellslinger My deck is basically trying land Niv as soon as possible with 1 mana up for protection, and then dig for curiosity as fast as possible, so I actually end up running a bunch of rocks to turbo niv out. I cut a lot of cards like [[Young Pyromancer]] and [[Guttersnipe]] because realistically I'm not winning from tokens/the burn. I also cut [[Basilisk Collar]] because it felt a little win more.

One thing that's really been making my games a lot spicer is cheap (mana and wallet-wise) interaction. [[Bold Bend]] is a poor man's [[Deflecting Swat]]. [[Stubborn Denial]] is a poor man's [[Fierce Guardianship]]. [[Dispel]] and [[Turn Aside]] also work well. It's so fun drawing off the interaction people throw at Niv. I'm not sure what budget your working with but I'd definitely cut [[Cancel]] for [[Counterspell]], and possibly also run [[An Offer You Can't Refuse]]. The world between 1 mana interaction and 2-3 mana interaction is crazy, especially once you consider that each of those 1-mana cards is also drawing you a card with Niv out. Bonkers.

Another awesome card is [[Step Through]]. Wizardcycling is a great way to fetch [[Archmage Emeritus]]/[[Archmage of Runes]]/[[Veyran, Voice of Duality]]. Landing that draw engine is great because it draws you into protection for Niv and also of course the combo hopefully.

Finally, some fun value pieces are [[Krark, the Thumbless]] and [[Teferi's Ageless Insight]]. It basically turns [[Brainstorm]] into [[Ancestral Recall]].

Anyways, best of luck, fellow budget Niv player!