I don’t know if it’s the cheapest but I recently got back into magic after 15 years and my friends have been playing eternal brawl.
It plays really well with 3-4 players but works for 2-6. Like commander you have a commander but only 60 cards and all of your cards need to be from a block to avoid bad combos.
It’s possible to set up a deck for a few dollars if everyone is avoiding expensive cards and using a few sets old block usually means most cards are a few cents.
@Doug @robotsheepboy
I'd say cube, especially if you print off the cards yourself. Then you are just paying for sleeves.