"Laid by the elusive cocoa bird. There's a surprise inside!"
Upon buying the Chocolate egg upgrade, 5% of your current cookie bank will be added to your bank, and will count in your "total earned". There is no upper bound, making this upgrade potentially the highest producing event. To make the purchase of this upgrade profitable you need to have a minimum of cookies (without taking discount upgrades into account), where is the amount of eggs purchased. This means that if Chocolate egg is the last (20th) egg you purchase, you need to have at least 1,162 trillion cookies in your bank to make it profitable.
A strategy could be to not buy the Chocolate egg upgrade at all during a run, but rather to save it. When players are ready to reset, they can sell all their buildings, and then buy the Chocolate egg. Likewise, during the end of a run, it may be wise to not spend ludicrous amounts on buildings, but simply keep the cookies in the bank (because selling them returns only half their cost). If the "Earth Shatterer" dragon aura is available, it may be a good idea to switch to it to take advantage of the 85% return on selling buildings.