1) Lead Dragon 5,000 10%
2) Copper Dragon 7,500 15%
3) Silver Dragon 17,500 20%
4) Gold Dragon 25,000 25%
5) Platinum Dragon 40,000 30%
Then change the safe illustration to a dragon on a hoard of gold. Have a different picture for each level - or not.
I would rather own a Dragon then a safe

Safe is a misnomer anyway - just hit withdraw and see just how "safe" you gold is.
