I’m curious where/when these phrases originated: (I know the meanings.)
More fun than a barrel of monkeys.
And
Cash on the barrelhead.
Googling didn’t bring up much that I’d put credence in because, well, it didn’t pull up anything on the SD. 
What’s the dope?
I’ll guess.
Monkeys can be entertaining. They were and are a major part of animal acts. If one monkey is fun, then a barrelful is the ultimate.
Barrels were often used as counters in rudimentary shops (which may have been in a tent, or a thrown-together shack). “Cash on the barrelhead” means that youput your money on the counter and collect your goods. (As opposed to buying on credit.) Many goods used to be shipped in bulk and sold right out of their containers. Pickles come to mind.
Searching for previous threads, using “barrel and monkeys” you can find http://boards.straightdope.com/sdmb/showthread.php?t=6606&highlight=barrel+monkeys where I cited 1895 for the “barrel of monkeys” phrase, which was also cited at that time with “bushel of monkeys.”
It was student slang at the time.
As to “cash on the barrelhead,” read Maven’s Word of the Day.
And then, if you “give 'em both barrels,” then you have cash and monkeys.
My personal “oh my god why didn’t I ever realize that before” story about a “barrel” saying:
First, you have to understand that I have been exposed to guns (old-style guns at that) all of my life. Both my parents shot black-powder muzzleloading rifle competitively. We were at shooting competitions every weekend in the summers. All of my parent’s friends were rifle shooters. My father built muzzle-loading rifles!
And yet, I was in college before I actually realized where “lock, stock and barrel” to indicate “the whole shebang” came from. :smack: