I always say this when I make budget videos. Don't focus on the cheapest cards, focus on the most expensive cards in a deck that is otherwise really cheap.

That may sound really counter intuitive, you want to spend less but I am telling you to spend more? But the key thing is that the vast majority of non-blue/black decks have 1 or 2 cards that are really expensive and the rest of the deck is dirt cheap.

So, for example, can you afford to buy a playset of Aether Vials? Depending on where you get them and condition you are looking at a cost of around 100 dollars. But what now allows you to do is build the vast majority of tribal decks in Modern which , outside of Vial, are really cheap - so now you have the major cost of a bunch of decks out of the way. The 2 major expenses in every creature deck are Vial and Cavern of Souls - and Cavern is something you should play...but don't NEED to play.

There is an alternative buyign strategy with the above and that is to focus on buying all the cheap pieces first and then buy the expensive pieces later. A classic example of this is Green Tron. You can buy the vast majority of a fully built Tron deck for about 50 dollars...you just need to play bad win conditions until you can afford the Walking Balistas, Karn TGC, Karn Liberated which are all still relatively cheap cards by Modern standards.

Where I would offer some advice is to be very careful with buying decks that are really cheap but the cards in those decks are specific only to that deck. The reason I say this is that if you get bored of the deck, a ban happens, the deck because completely unviable, then you have all of these cards that have no trade value.

For example, Enchantress. Outside of the lands most of those cards are not worth much. You will also struggle to trade them because who wants 4 copies of enchantresses presence?