I would probably have three separate addresses, player 1's collateral, player 2's collateral, and the kitty, which is the money that the winner of a hand receives. Each player would probably need to deposit something like $5000 into their collateral address, which they receive back assuming they do not break any of poker's rules. But if either player issues a challenge, whoever loses the challenge loses their collateral and forfeits the kitty to the winner.

Assuming fair play, everything happens off chain except for the deposit and the withdrawal. Those are ordinary-sized bitcoin transactions so the cost would be the cost of two regular bitcoin transactions. If there is foul play, whoever tried to cheat loses $5000 + the kitty. The $5000 pays for the fees of producing the fraud proof.