Since the rewards chests are similar to the winter event the probability chart from the winter event can also be applied to the easter event.
Where "flakes" is written read "sky essence".

Calculations are made by fasdfTTT from the US forum and you can find the source here.

The expected maximum "free" gain of Sky Essence is aproximatly 4600 sky essence. 980 from the daily quests (28*35), 840 from the random spawns (28*+-30 per day) and 2787 from the main quest.