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Extort triggers on a spell being cast. Thus it is put on the stack above the triggering spell and resolves before it. Each extort ability triggers seperately, so with two extorters out, you get the choice to pay or twice and can extort for up to 2 life per spell. This cost is paying mana, not tapping lands. Tapping a land for its mana ability will produce the appropriate mana which you can then spend on spells or abilitiues (like extort).