Oh, good to know. You've understood perfectly how it works, but there must be a bug. Bronze coins normally increase hp healing, not gold gains. It's gold coins that increase gold gains (just like green coins, which improve everything). I'll check the descriptions, effects, etc. anyway. I'll look into it and fix it.
But if you had gold coins, then you should have made a profit. (That's why it costs 120 gold coins when it only yields 100 gold coins in base value.)
Thanks again <3
Altego