On my second account, which is on level 67 and i don't play much, i decided to make some tests.
So i sent my Epic Fire Dragon to quest for coins for about 20 - 25 times in a raw and the results were: 1500 / 2000 / 2500 / 3000 coins.
Perhaps there are one or two better earnings, but it seems that these are the most common ones and i haven't seen anything better untill now.
I think the "quest for coins" exists for helping new or low level players to earn some extra coins.
So, i made some quick maths.
The cost for this quest is 1000 coins each time and lasts 6 hours.
The Big Red Rocks habitat can hold up to 2500 coins.
A 10 level Fire Dragon earns 1968 coins in 6 hours (On level 15 earns 2286 coins).
The Dragon, during the quest can't produce coins in the habitat it belongs.
So, by starting this quest, you are losing 1000 coins (directly) and 1968 coins (indirectly), which means ~3000 coins in total (~3250 on level 15), to earn at best 3000 coins!!!
Even if we quest for crops, the worst result will be 3000 coins, which is the best common result in the especially coins' dedicated quest!
When it was quest for gold, i could understand the coin fails to be low earnings, but now what's the point?
Conclusion: What's the need of this quest in this form? It "sounds like a bad joke to my ears" and i don't know what the TL developers were thinking when they configured it this way!
So, it has to be re-configured, to really helps players who need coins (with some normal and extra, not so common, amount of earnings) and not to be such a waste of time like it is after the last changes