Hello.
There is no secret: the average of the gains compared to the ranges of gains of the missions is around 50%.
It is true that it is a bit light as coding on the RNG of the gains.
Normally, at the end of my day, I had the possibility of earning around 40k starbits but I very rarely exceed 20k.
By organizing your missions differently and varying your ego level, you can increase your earning ranges.