Quest isnt worth doing. The final quest is to buy 3,000og which is the premium currency. This will cost you 10$. The reward for the quest will barely get you 10$ in Steam/Amazon currency.
I'm not sure if you can exchange Kama(the general currency) for OG and it count, but it will cost you 2,079,000 kama. Which at my rate from lv1 - lv35 I obtained about 8,000...
Gamekit labels the game "Free" but it's in fact not free.