Depending on your prefered format, patience and number of games you play it can be actually free or it can cost you thousands od dollars.

I spent around 20 dollars since I started playing in 2019. I have practically all cards for standard format i want to use + bank of gems and gold to purchase new set when it comes out in September.

Potentially more expensive as you can't buy singles in Arena.

If you're looking for a free version of Magic, there is Forge. It's a FOSS engine for MTG with multiple game modes, including an adventure mode which is extremely reminiscent of the old Microprose Shandalar game. It will even automatically download the real card art as you come across it in game.

It's under constant development with the latest cards and has multiple updates per week. I primarily play adventure and in the six months or so that I've been playing, there have been huge content additions such as quests, boss dungeons, new enemies, graphics etc

Just be warned that it is dangerous to your free time to have this available on your phone at will.

I played it for months and received enough free cards to have several decks, including multi color ones, ready to go.