Credit drops seem to use the same formula as item drops. Going through some logs of the gangers - there is a chance of any credits dropping, and if successful, it's an even distribution between 5 and 25. With a perception of 10 or lower, I get any credits about 2/3rds of the time; with a perception in the 30-40s, I'm getting credits to drop about 95% of the time; with even higher perception, some credits always drop. Perception doesn't seem to affect how many credits drop if any do.
(Thanks to Barton26 and PenguinPyro and anyone else I'm forgetting for sending drop data!)