Man the most I've managed to get my coin bag so far was 3000, but to get to 1000 coins, first you have to find a green gem and fuse it with the coin bag and then you have to find a red gem looking broken and fuse it again with the bag to win the 3000, they are somewhere in the desert maybe, I don't remember very well where I got it because I left Nel in automatic mode fighting for me then I cleaned the special chests on the map, sorry I can't help much more.