Interesting topic.. I think I can contribute something here
Being that I work in the Federal Reserve Bank, where I process currency, I can attest to how filthy money is. My hands are constantly black from the grime of the bills. I have hand sanitizer that I use every half-hour or so to keep my hands moist and kill some germs.
As far as the cocaine residue on $20's.. let me give you some insight... At the Fed (where ALL of your money is bound to end up sooner or later), we use the SAME MACHINES to process all the denominations (1,2,5,10,20,50,100), and the machines get a quick once over with the vacuum and air gun between shifts (and sometimes during a shift when dirty money causes jams). Not only that, but the singler mechanism is pneumatic (which means air is blown onto the top of a stack of bills to single them into the transport). This sort of cleans the money a bit. There is so much dust and debris, that it really is a rather dirty environment. So that means that ALL currency will have residue on it. Now there is such a small amount that if you were to start sniffing bills, you would most likely get pneumonia or some other respiratory disease faster than you would get high.
Think of it this way... a LOT more americans wipe their asses than sniff cocaine. Some people don't wash after either, so guess what that means. I suspect that there is more fecal and urine residue on currency than any kind of drug.
Still wanna sniff that $20?