At first, Alister had planned to make the sharing of faith energy to 20% only. But then when he realized just how much faith energy he needed in order to become stronger, Alister changed his mind and increased it to 50%.
Anyway, he knew that most of the grand-level powerhouses would accept his offer even if he made them contribute 50% of their faith energy to him. After all, 50% might be a lot of protection fee to pay, but it's still better than not becoming a god and having no ticket to the God Realm.
Fifty percent of 2 billion was 1 billion. Losing 1 billion might be painful, but not earning 1 billion was more painful.
At least, looking at the bright side, Alister wasn't greedy enough to take 90% of their faith energy.