The most expensive liquid in the word is…
Scorpion venom at around $39 million dollars per gallon. That’s about $2.4 million per cup.
So why is scorpion venom so darn expensive anyway?
When electrical stimulation is used to “milk” the venom glands of scorpions, an average yield of anywhere from 0.006 mg to about 2.0 mg of venom can be obtained from a single scorpion. 
With one scorpion only able to produce such a teeny tiny amount of venom at one time, filling
up even one cup can take ages! It’s a very time consuming process that requires the use of A LOT of scorpions.
Now on to an even better question. What on earth do you do with scorpion venom? Scorpion venom is used for medical research, most recently to see if it could possibly be used as a cancer treatment and so far, things are looking pretty optimistic. Check out this article, On The Horizon: Scorpion venom as cancer treatment for more info.
So if you’re ready to get rich, grab a few friends, find some scorpions, say your prayers and milk away!