k first of all i haven't fully tested this but i just started assuming this.
it seems that you get more gold and purple drops when you have less cash with you (doesn't matter how much you have in storage).
when i run with 1-2k on me i get at least 1-2 golds and 3-4 purples each hydra run.
when i have 10-20k on me i hardly get a purple and almost no gold drops.
testing this some more and keep you guys updated