I'd recommend setting up a gold -> platinum NPC trade.
A lot of users are asking for it (I saw this on iPhone forums as well).
As long as the exchange rate is quite high in order to avoid abuse of the system (for example, 1 plat for 4k gold), it's a win-win situation.
For now, if you want to enjoy the full game (at least dungeons/campaigns), you've gotta shell out at least $20, which is kindda on the ludicrously high side for an Android game.