Comment by strstr

When it comes to playing the game between friends outside official tournaments, you are basically correct (though some use cost as a power level limiter).

When it comes to trading, you don’t want to accidentally pay a premium for something you won’t be able to resell. Lots of players view trading as, more or less, leasing cards. Valuable cards typically have fairly stable prices (though there are notable exceptions). Buy for a dollar sell for somewhere between 0.75 and 1.25.