All player values are calculated every Sunday. The calculator goes through all players in the game. For each player it will search for similar players who have been traded recently. These similar players are found by comparing age and sum of stats with other players.
When all similar players are found, the calculator assesses the average of these players in relation to the player's own stats sum. The relation between the player's own stats sum and the similar players' average stats sum is multiplied with the average value of the trades of the similar players to get the market value.
The player's value does not necessarily reflect the market value. There are certain players, where the amount of trades are so small, that the calculator returns a value that is skewed compared to others. You should not therefore trust the players valuations blindly.