I think the odds were already shared here and it was Marquez who is the favorites, though it's not official yet at least having an idea is a big help to determine who is the possible favorite, but then the odds difference is not that huge so it's like a fair or even match and bettors like that.
Yes, I have not seen that there were already official odds for this match. It is still quite far before this match happens. So, those odds that we see coming from unofficial people then expect those to be guesses only from a fan or any analyst that monitors this match.
AFAIK, both fighters have become champion during their pro boxing career, and both have fought the greatest in the boxing world, so we have to expect that they will be both respected and both fighters will have their fans to support them, so it's probably a 50/50 fight if one is a favorite here, for sure just a slight favorite or there could be line changes as money will come on both sides.
Meanwhile, there are some boxers who did not appreciate these exhibition matches as they feel people are not getting the right entertainment and the real boxing which is run by the pro is being affected.
Just imagine, if Mayweather gets $100 million in an exhibition fight but other fighters are even struggling to get $10 million in a fight, they'll feel frustrated for sure.
You can't really compare Mayweather (I'm not a fan of his), however, give him credit, as much as we hate him because of his brandish and arrogant style, he can back up his talks, he is still undefeated and has beaten a lot of champions.
And so with his popularity commands huge amount of money, vs Pacquiao, vs Conor, two of the biggest PPV in history. So if other fighters to get that huge paycheck, then they have to take the road of Floyd.