In my latest game I'm bringing in 500/turn so if I'd saved it since I started making this much early industrial I'd have about 500*100 = 50,000. How did you get so much? Did you just never use it?
Around zero. In CIV V money is meant to be spent as you can't get any interest from money in the bank unlike Civ III.
Although it is handy to have some cash ready for purchasing 1-2 units in case you get backstabbed.
I play at Emperor-Deity
Cash in the bank in Civ5 actually is taken into account by the AI when they decide whether they want to DoW you or not. So even though you don't get interest on your cash, you still get a benefit as you don't have to pay unit maintenance. And of course you have the tactical advantage that you can buy the newest unit right where it's needed when the DoW comes.