That kind of money going to a fighter simply doesn't exist in MMA. I know that this would be a boxing match, but the UFC is still going to insist on being a major player in the promotion of the fight, and they don't pay fighters anywhere near those amounts. Even McGregor probably barely hits 8 figures, if even makes that much (he got $3 million for the 2nd Diaz fight, and even that number lit the MMA world on fire and has created big problems with the other fighters on the roster). The UFC heavyweight champion just made $690K to defend his title this past weekend. Yes, they probably make more in undisclosed income (bonuses and PPV points), but it's nowhere remotely close to what Mayweather makes even in his least lucrative fights.
That is the problem that I see with getting this fight signed. Floyd will obviously insist on getting the $ that he always does, and will insist on handling the promotion as well, like he always does. The UFC has no incentive to let McGregor fight unless they are promoting the fight and controlling things (because after all, it's the face of their organization that's going to be made to look like a fool if it actually happens), and they can't give Floyd $100 million or something and then sit down at the table with a Stipe Miocic, a Michael Bisping, or a Jon Jones (if he comes back) and go back to offering them a million or so (or less). It would create even more chaos and bad blood among their fighters than is already present. The UFC actually still has top-10 fighters fighting on the main card of PPV events making like $20K -- I just don't see any way that they will be willing to make Floyd an offer anywhere near what he is used to, or what he would accept. And if they did, it would create more problems than it would solve for them in the long run.
ETA: I absolutely agree that it is brilliant promotion by both sides, keeping both guys names in the media and generating interest and clicks for both of them, while actually doing nothing. Both sides can easily back away, too, without any harm, by just blaming the other guy for wanting too much $ / not offering them enough. Good business by all parties, even if it never happens.