Win Gold Bars in Milestone Levels
For motivation, gold bars can be given out at milestone levels. For example, every 100th level one can receive 5 gold bars, every 500th level one can receive 10 gold bars, and every 1000th level one can receive 20 gold bars. Of course these numbers can be adjusted and don't have to be final, but you get the idea.
This idea hits two birds in one stone, one being a way to get gold bars without money, and the other being it would encourage and motivate players to keep on playing. Tell me what you think about it!