EXCLUSIVE

Mel Gibson's baby mama, Oksana Grigorieva﻿, just got word from a California appeals court ... she made one of the worst business decisions in the history of business ... because they just ruled she's only entitled to 1/60 of what she could have gotten from him had she just shut her trap.

The California Court of Appeals just dropped the hammer, saying Oksana screwed herself when she went on Howard Stern in 2013 and claimed domestic abuse at the hands of Gibson. The actor was on the hook for $750,000 -- a settlement he had struck with her -- but had only shelled out $250,000.

The agreement said she would forfeit the balance if she blabbed about her allegations, and the court said that's exactly what she did when she went on Stern. So, Mel does not have to pay the $500k balance.

Mind you ... Mel had agreed to pay Oksana $15 million if she kept those horrible audiotapes secret, but she changed her mind thinking she could get even more and disavowed the deal. Instead of $15 mil, she eventually struck a deal for $750k ... and that has now been whittled down to $250k.