If your a veteran at Timeless Isle - this quest is instantly over. That means you will already have the coins in your possession from everything listed below.
If you're NEW to Timeless Isle - and just starting this quest - there are many ways to get the coins... Here are some tips to speed this process up very quickly:
#1: The tiny dirt spots in the soil - contain some coins. Pick them all up.
#2: Have a skill like skinning? Herbalism? Mining? They all yield some coins...
#3: There are various chests around the island - some are repeatable (meaning you can grab them again) and others are just a one time deal. The ones that are repeatable are:
-- The one on top of the pillars in the tiger area
-- The one you get to from running along the ropes
-- The cache on the ship at the far end of the isle beyond the froggies
-- The crane/statue thingy at the top of the hill where Cinderfall is.
#4: Do the daily quests. There is a quest giver on both sides of the arena. A quest giver in the tiger area near the caves. ...and as you kill cranes, tigers and turtles they will drop a special meat that a vendor will want. She'll reward you with coins.
#5: All mobs drop coins usually.
#6: You can gamble with the chests in one of the caves with the big monkey - although this isn't recommended. You'll probably lose more than you make - however I did get 10K coins from one chest ONCE so you *might* get lucky...
I think that's about it - there are guides on the Timeless isle as well that go into more detail about locations/strategy/etc so make sure to check them out.