...So let's consider Bank of America alone, with 5,700 locations and 16,000 ATMs.
This says an ATM currency cassette holds 1,000 or 1,500 bills. If we assume the average ATM has the smaller size cassette and it's a quarter full, that's 250 $20 bills, or $5,000. 16,000 ATMs at $5,000 apiece equals $80 million sitting just in Bank of America's ATMs. Then if we assume a bank vault has $1 million in it, that's $5.7 billion just for Bank of America's locations alone. Then, of course, Citibank has 3,300 regular ATMs, an ATM in 5,500 7-Eleven stores, and 1,000 bank locations in the US. Wells Fargo has 4,100 bank locations and 6,800 ATMs. The point is, there's likely to be way more than $6.2 billion in cash floating around...
Here is some info from someone (me) who used to fill up ATMs and drive/ride on armored trucks.
The typical ATM cassette actually holds 2,000 bills. That's the standard size. The 1,000 or 1,500 bill cassettes are for those tiny free-standing ATMs in some convenience and liquor stores that look like you could probably pick it up and walk out with. They are typically not associated with large banks like Bank of America; usually, they are operated either by the store itself or by smaller local banks or credit unions.
Furthermore, those smaller ATMs actually hold two or three cassettes; the regular (large) ATMs hold four. In almost all cases, about half of the cassettes (one or two in the small ATMs, or two or three in the large ones) are filled with 20s, and the other may have 10s or 50s. Some large ATMs are configured with one 10, one 50, and two 20s. When filled, the 50 cassette is usually not actually completely filled with 50s; it may be only half or a quarter full. So, a typical large bank standard ATM may contain, when considered "full", well over $100,000. It was not unusual for a large bank's ATMs to be refilled when they were down to "only" $10,000 (in almost all refilling situations, the old cassettes, with whatever they have left in them, are removed, and replaced with filled cassettes; the cassettes are emptied, counted, and filled to full-level by other people at an ATM service company's office).
However, I do not believe a typical bank branch location has a million in cash in its vault (or total cash on hand, excluding what may be in its on-site ATMs, which usually cannot even be accessed by the people that work at the branch). My wife, who has worked at a few different branches of two different banks, says the average bank vault probably only has about $200k to $300k.
But don't forget the bank depots. There are several bank depots in Baltimore, including one that belongs to Bank of America. A depot is called a "hole" by those in the armored truck business, because you back your truck up to a loading bay inside a garaged area that's patrolled inside and out by shotgun-toting guards. The hole is where the bank company receives large amounts of cash from the Federal Reserve, re-packages it and distributes it via armored truck to its individual branches. It's also distributed to ATM service companies, who will pick up the bank's cash from its depot, take it back to the company's office, and fill up the ATM cassettes there. I made those runs a number of times. I once held a million dollars in cash in my hand, in a shrink-wrapped package; we picked up three of those and took them back to the office (that was the most amount of cash I was ever in direct contact with).
The neat thing about a hole is you get what you're getting from basically a teller at a window. The window is bullet-proof glass (or whatever material it actually is); it appears to be about three inches thick. The actual neat part is that if you look over the teller's shoulder, behind them, in the processing area are pallettes and pallettes of bundled up cash just sitting there. Pallettes are, I guess, about three feet by four, and they're stacked with neatly bundled bills in a stack about three feet tall. We're talking about 36 cubic feet of cash here, and that's just one pallette!
The Federal Reserve branch office in Baltimore (on Sharp St. if anyone cares to pay a visit) had about twelve of these, that I could see. Some other intrepid member could try to do the math and figure out how many bills that is, by volume...I'm guessing the bank depots (the holes), especially the Federal Reserve branch depot, had to have well over a billion dollars sitting inside them.