When you defeat a combat enemy, your highest stack of coins is collected. I agree with many of the points here regarding the pacing, random bad luck that is insurmountable, etc., but it hasn't stopped me from playing over and over so I guess they are doing something right. The only tip I have to offer is to spend on the Digger upgrades (coin runes) early and often. Not only does it give you the obvious benefit of increasing your income (the first purchase doubles it), it also helps make the rune-changer items much more powerful. When your coins start at 3 or 5 or whatever, you will always have stacks of them laying around that you can convert to swords/shields/etc. to great effect.