While this tends to be pretty obvious when the cost is paying Energy, it's less clear if the cost is performing some other effect, like discarding a card or removing Presence.
While you must be able to pay the cost in order to get the bonus, ongoing effects can alter the final result of the cost being paid. For example, you can successfully pay the cost of "Destroy 2 Presence" even if Flowing and Silent Forms Dart By will end up saving that Presence from destruction if you do so. On the other hand, you can't attempt to pay the cost with Presence under Absolute Stasis, since that Presence only exists for Victory/Defeat – it doesn't exist as far as the Event is concerned. Tags: Action Modifiers, Flowing and Silent Forms Dart By, Absolute Stasis (Fractured Days Split the Sky), Turmoil's Touch (Wandering Voice Keens Delirium), 11 more...
For instance, Gift of Living Energy (a Minor Power) wouldn't count it towards "If you have at least 2 Sacred Sites...", nor would Fear of the Unseen (a Fear Card) treat it as a Sacred Site for scaring away Invaders.
Note that if you Entwine with River Surges in Sunlight, their River's Domain Special Rule only applies to them (read it as "counts as your Sacred Site"). So for targeting, they would treat any Wetland where either of you have Presence as a Sacred Site, but you would still need 2+ Presence as usual. Tags: Sacred Sites, Presence, Entwined Power, River's Domain (River Surges in Sunlight), 6 more...
For instance, Downpour Drenches the World could choose its second growth option (which has 2 "Add a Presence" icons), add 1 Presence to uncover a "Move Presence 1" space, use the Move 1 to shift a Presence, then place a second Presence from the new position. Or Many Minds Move as One could gain Energy with its 3rd Growth option, then use it to gain a Power Card before deciding where to place Presence.
Discarding two cards is not listed as a requirement or cost, but as a Growth effect. You must perform that effect to the extent possible. (In this case, you would be left with no cards to play.) Tags: Growth, Downpour Drenches the World, Ember-Eyed Behemoth