Weekly heroic or run a bunch of strikes to get a ton of engrams. Save as much as you can see decryption. If I have like 10 engrams and decrypt them at the same time I average at least 1 coin. Also you may level up the cryptarch to get more from the postmaster, which could be coins.
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This happens to me too. I will do a few strikes than go to tower and turn in 10+ and usually get a mote of light and one strange coin. Also seems like if I have two blue engrams of any kind the second seems to drop a coin more often out of it when turning it in to rahool. Also I know there is atleast one gold chest on I believe Mars that gives u a strange coin. Might be another gold chest on one of the other planets too. I just can't remb as it's been so long since I got the gold chests.