Lookin' to make some extra cash during the Floyd vs. Conor fight?? BET ON THE BOOBS ... 'cause Demi Lovato's twins highlight the fight's list of prop bets!!

The cyber-gambling site BetOnline.ag -- one of the biggest gambling sites on the net -- is offering up a bet on whether Demi will show cleavage or cover up the goods during "The Star-Spangled Banner" before the fight.

As of right now ... the odds are definitely pro-boob:

Yes +350 (bet $100 to win $350)

No -500 (bet $500 to win $100)

There's more (not as sexy) bets ... like over/under 1 min 52 seconds for her rendition of the anthem and whether the Biebs and Lil Wayne will make an appearance.