Why not both? I would say the major cost of the game is time, energy, and effort. It requires imagination, as it's not a virtual game and you need to use theater of mind for a free game or spend some money to get miniatures and models.

You don't have to purchase D&D like you do a console game. I guess the best way I could relate the two is that you may possibly consider the modules and source materials as "DLC".

You don't necessarily need to buy anything to play the game, as the basic rules are readily available online for free. Honestly, if you don't have money but have plenty of time you could find and organize everything you need and want for free to shape your game.

Purchasing things is all optional, but could possibly provide better all-in-one source materials and make your game experience easier when trying to reference different rules outside of the basic starter guides.

Everyone here would obviously recommend the Players Handbook, Dungeon Masters Guide, and the Monster Manual. There are so many different modules outside of these that come with alternate optional rules that you can choose to incorporate into your game.

For example, the Players Handbook lists a variety of races and classes, but that doesn't mean you have to use them all in your game. Maybe in my world there are no humans remaining, and the party's main quest is to find out what happened to them.

You can purchase "quests" like Icewind Dale or Curse of Strahd and play the whole thing, or you could take bits and pieces and incorporate different aspects that you like into your own game.