Quoted from CrazyLevi:

It means Bill of Materials. It is the exact cost (not counting R&D, advertising, overhead etc.) of the materials that go into the production of a single pinball machine.
BOM is a closely guarded secret that NOBODY outside of the industry actually knows to any degree of certainty. A BOM has NEVER been released publicly to the best of my knowledge for any machine in the history of the industry. Nobody really knows what the BOM of any machine is, or how much higher or lower the BOM of any machine between models or manufacturers is.
That being said...EVERYBODY here is a BOM expert. People generally think they know what a game costs, what parts cost, and how much higher the BOM is between models and manufacturers, and base lengthy rants and arguments on the imagined BOM.
I generally refrain from BOM assumptions as I at least like to attempt to give the appearance that I have some idea of what I'm talking about, but this little detail doesn't seem to matter to most folks.

Quoted from NimblePin:

Dude,
Why are you yelling?
Are you still coming down from that Beyoncé concert?