I'm looking to play casually (in person) with some friends and we were wondering what the cheapest format to start buying for would be? And what might be some good pick ups (we have the starter decks but want some more variety) thanks
Pauper is a very cheap way to play Magic, 60 card decks with commons only. Some of the top competitive decks only run about $30, and you can build fun decks up with 10 cent cards from your LGS.
For casual play, my favourite is still Commander though. You can definitely build commander decks on a budget (one of the ones I made run me $26) and the format is a lot of fun!
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.
Pauper is the obvious answer. It's unique in that every card in the format is Common rarity, so that mitigates price already. Check out MTGGoldfish's Metagame page for Pauper, which lists competitively viable decks and their price. Not sure how up to date it is, but it's a good starting point!