Sardonyx only gives finite life to creatures, not conjurations.
As a Necromancer, you have to pay to reanimate your eternal servant when it gets destroyed. You can't wait.
You have to pay the cost for Rise Again when the creature gets destroyed. You can't wait.
The wording on eternal servant is less clear than the wording on Rise Again, but the timing on the payment works the same way assuming that Rise Again is face up.