If an opponent then plays an effect that grants a strike (e.g, Aid from Bats), they pay the additional cost.
Adding costs to a strike or strike card
Rulings
Strikes are paid at declaration. If an ally spends their last life on a strike, combat ends before strike resolution. If the strike damage depends on the amount of life/blood of the striking minion, it is calculated after the strike is paid for (e.g. Shambling Hordes).