Juan Soto has 2 more pre-arb years and 3 arb years. Let's say that's a total of $50 million based on a standard $12/16/20 arb. Then add $30 million a year for every free agency year you want to buy out, as a starting point.
8 years, $140 million.
10 years, $200 million.
My favorite - 15 years, $350 million to be the biggest contract in history, make him a lifelong Nat, and probably end up being a bargain when you account for 15 years of salary inflation.