The cheapest competitive constructed format is Standard, with prices between $200-$400. https://www.mtggoldfish.com/metagame/standard#paper has the current decklists. However, standard rotation is happening in about 2 months, so if you were about to get back in, I'd wait until Throne of Eldraine comes out on October 4th. (or the prerelease, the weekend before, which is always great fun)
Limited formats, such as draft, are much cheaper, with a draft being only $18 for three rounds of play. Each player opens a booster, takes a card, passes left, repeat until the cards are gone, takes the second, passes right, etc. for three boosters. It's much cheaper, more repeatable, and doesn't require an up-front investment. (Plus you keep your cards.)
Limited formats, such as draft, are much cheaper, with a draft being only $18 for three rounds of play. Each player opens a booster, takes a card, passes left, repeat until the cards are gone, takes the second, passes right, etc. for three boosters. It's much cheaper, more repeatable, and doesn't require an up-front investment. (Plus you keep your cards.)