Honestly, I would just grab the Modern Event Deck. It’s not a top tier deck , but it’s affordable and will let you figure out what style of deck you want to build. Also, it comes with cards that are always useful like Path to Exile, Inquisition of Kozilek, Elspeth, and Sword of Feast & Famine. It’s a very good value (value of cards ~$80, deck costs $55), so you should be able to use the contents to trade into cards you need for whatever deck you settle on. If you want to see what other decks are popular and approximate costs, check mtgGoldfish. The absolute cheapest is burn at $200, but a lot of those cards are things you can trade into with the Modern Event Deck if you decide that’s what you want to run.
P.S. I listed Amazon because Prime members can get it in 2 days, and students can get 6 months of prime free.
Given the reprint of fetchlands, those trying to get into modern can look at UR Storm and UR Delver as budget entry. Without fetches, they are probably $75 and $150 respectively, with the later being Snapcaster Mage which is probably an okay card to spend $25 on.