This is what they're fighting for -- in addition to the $400 million -- the official "Money Belt" ... and it's LOADED with gold and diamonds. 

We spoke with a rep for the World Boxing Council (WBC) who tells us the belt was specially made for the McGregor vs. Mayweather fight -- and they spared no expense! 

We're talkin' 1.5 kilograms of 24k gold (that's roughly 3.3 pounds). 

3,360 diamonds placed throughout the piece -- with an additional 600 sapphires and 160 emeralds!

The strap is made from crocodile leather.

It's not the first high-priced belt the WBC has made for a Mayweather fight -- they commissioned a special "Emerald Belt" when Floyd fought Manny Pacquiao ... which was valued at $1 million!