I came across this fascinating graphic this morning. While it's probably common knowledge for those living outside the United States, soccer players can be bought and sold mid season if the price is enticing enough to an owner. Nothing like this exists in major American sports where players are under contract unless a trade can be arranged for equal or better return of players (and/or draft picks).
The biggest sports star in America is Lebron James, a professional basketball player. This has me wondering; what is the free market price one of the billionaire owners would pay the Cleveland Cavaliers owner to pry James away from their team? My guess is that it would rival or exceed the estimated transfer value for Lionel Messi because James has the ability to instantly add $300 million (or more) to an NBA team's value (the L.A. Clippers just sold for $2 billion).
As the graphic says, everyone has their price.