Tinfoil hat rumor:
So there was a guild mate who swore you could "tell by the last tier's boss' health how much the reward would be" he said if it was a 5m or 10m reward the health would be much more. Since you can retry over and over by retreating you could try this.
Since then I have been paying very close attention to the health ( but not retreating) ,but as luck had it I have only had 2.5mil reward since I heard this about 2 months ago.
Probably shenanigans but I sure would like to know a trick! So I'm keeping an eye on it....
