Don't hang on to the exampled figures & rates I gave, it's just an example! Set your minds free and look at the concept.
I also thought about the inflation. But maybe you or other player could come with ideas around this AAA-Bank. We could introduce an other event besides the bank robber. The bank could burn down, losing x% of your money. Or the bank could supply an insureance for your money with a fixed amount of premium
so in case of fire/robber your GC are save or at least a percentage.