"One fast way to earn money fast other than opening treasure chests is to catch more monsters! Its that simple. Even if you have no back-up slots available, the caught monster will burst into coins and you will also be able collect its monster data. If you DO have empty monster slots, releasing them will also earn you more coins and if you're lucky, other goodies too. Releasing higher level monsters will results in higher chances of more loot it drops."
"You can also get more coins when overrolling a monster. The more damage you overroll a monster with, the more valuable the loot it drops!"
"Useful Hints: Challenge yourself by first reducing a monster's health to near death, then overrolling by a high value during your next turn for greater loot drops, coins and health!" (TinyDiceDungeon.com).