'Value' is an intrinsic property of a piece in a given context, reflecting how much its presence contributes to winning the game. 'Cost' is a synonym for 'price', and these are arbitrary quantities that could be specified in rules that allow you to buy pieces from a budget to select your army. They imply the possibility to trade the pieces against some 'currency' that exists outside the board.
'Value' is an intrinsic property of a piece in a given context, reflecting how much its presence contributes to winning the game. 'Cost' is a synonym for 'price', and these are arbitrary quantities that could be specified in rules that allow you to buy pieces from a budget to select your army. They imply the possibility to trade the pieces against some 'currency' that exists outside the board.