Some costs include an {X} or an X. See rule 107.3.
April 27, 2018 · Dominaria
If a cost or effect allows a player to pay an amount of life greater than 0, the player may do so only if their life total is greater than or equal to the amount of the payment. If a player pays life, the payment is subtracted from their life total; in other words, the player loses that much life. (Players can always pay 0 life.)
September 30, 2011 · Innistrad
If a cost or effect allows a player to pay an amount of life greater than 0, the player may do so only if his or her life total is greater than or equal to the amount of the payment. If a player pays life, the payment is subtracted from his or her life total; in other words, the player loses that much life. (Players can always pay 0 life.)
June 17, 2011 · Commander 2011
If a cost or effect allows a player to pay an amount of life greater than 0, the player may do so
February 4, 2011 · Mirrodin Besieged
If a cost or effect allows a player to pay an amount of life greater than 0, the player may do so only if his or her life total is greater than or equal to the amount of the payment. If a player pays life, the payment is subtracted from his or her life total; in other words, the player loses that much life. (Players can always pay 0 life.)
October 1, 2010 · Scars of Mirrodin
If a cost or effect allows a player to pay an amount of life greater than 0, the player may do so
July 16, 2010 · Magic 2011
If a cost or effect allows a player to pay an amount of life greater than 0, the player may do so only if his or her life total is greater than or equal to the amount of the payment. If a player pays life, the payment is subtracted from his or her life total; in other words, the player loses that much life. (Players can always pay 0 life.)