1) use up all the free offers on tapjoy for platinum. i payed for sandstone cave that way lol
2) potions are cheaper when bought in mass, so try to only get the 100 potions option.
3) if having trouble staying alive, either change classes to one easier for you (if being constantly hit is your problem, try a mage or bird so you can kite the enemy from afar), change stat preferences to one easier for you (by going into dex or in, you can use staffs or bows on your bear. dex is actually one of the preferred stats) or revisit lower dungeons to get more monies and xp easier.

since now all the main maps are free, all you might want to get are 50 more stash slots, and 100 inventory, and maybe another character slot down the road. all can be payed for with the free platinum options.