well, if they were all that common, they wouldn't be valuable. At that rate, 5000 mobs would be about 10 insignias give or take depending on luck.

I grind 5000 mobs and lucky if I get a single SES. Other days I grind that many and get 4.
Depends on the rate of gamma rays from the sun combined with the alignment of the planets to the galactic core.

In other words - some days you are lucky, some days you ain't.