No matter what, it stands to reason that marijuana would make tons of money. I know more people who use marijuana then almonds :)
The reason that people report this erroneous fact regarding marijuana has to do with a simple fallacy equating street value with crop value.
For example, it has been said that there is only 4 cents of corn in every box of corn flakes.
Likewise, the street (or store) value of marijuana is mostly things like marketing, distribution costs, etc... To get the actual crop value that is paid to growers, you would need to divide by a lot.
In conclusion, marijuana is not the most valuable crop of California, or any other state for that matter. It is is not even close.