It is the cheapest to technically just play - you just need a ball. You can set up shoes as goals or whatever you want. However, in the U.S., to become a great player you need coaching and coaching is extremely expensive at the highest levels. A local youth club near me charges $3,000 per player per season (3 seasons, so $9,000 for the year). If you want to be seen by college coaches and play in showcase tournaments, etc. you need to play on those elite teams. College coaches don't pay any attention to high school teams because high school soccer is abysmal.
When I was playing in the 2000's, clubs cost about $1500 - $2000 per year, and prices have skyrocketed since then.