The amount of life an opponent needs to pay for that spell or ability to resolve is equal to Phyrexian Fleshgorger's power at the time the ward ability resolves, which may be different from its power when the ward ability triggered. If Phyrexian Fleshgorger is no longer on the battlefield at the time the ward ability resolves, use the power it had the last time it was on the battlefield to determine how much life that player must pay (though if Phyrexian Fleshgorger was the only target, the spell has no legal targets anymore and hence it would not resolve even if they pay the life).