There are 5 levels of monsters: Common, Uncommon, Rare, Legendary and Unique. As the ranks go higher, the chance of a coin dropping from the battle gets lower. The chances are:
Common - 50%
Uncommon - 35%~25%
Rare - 15%~12.5%
Legendary - 10%~5%
Unique - 4%~1%.
On certain days there is a bonus to the drop rate. On Mondays the drop rate is increased by 1.2 times its normal value. Other bonuses can raise the drop rate by 1.5 times.
Did you know? Edit
Event towers with unique monsters have X3 drop chance and most only drop coins every 10 floors (10, 20, 30, etc.).