A model that converts chip stacks into expected prize money.
In a tournament, doubling your chips does not double your expected prize money. That's why calls need more equity than in cash games.
At the table
With 3 players left, prizes of 50, 30 and 20, and stacks of 5,000, 3,000 and 2,000, the player with half the chips has an expected prize of about 38, not 50. The three expected prizes are about 38, 33 and 29, closer together than the chip shares.