It’s not the absolute cheapest, but I recommend trying out commander if you have not already done so. The value of the preconstructed decks is very high as they come with a bunch of useful rares. Some of these decks are as low as $20 on Amazon. Additionally, almost every card ever made is allowed to be played, and nothing ever rotates out like in standard format. It’s a casual format so if you don’t have a lot to spend on perfecting a deck, many people have several tiers of decks for more fair matchups. If it’s just you and your friends, most of the preconstructed commander decks are well matched. Also there are 100 unique cards in each deck, so there is a ton of variety even in one deck and it will take many games before you even see all the cards you have.