The odds of any 71 year old being alive and healthy in six months is way less than 98%, so that should be enough to make you back off right there. He could suffer a stroke or be disgnosed with Alzheimer's that would force him to withdrawal..
Additionally, Mccain would be a far greater risk than your average Joe as he is running for President and a potential target for a deranged individual. Lastly, some additional skeletons could come out of the closet that could cause him to resign. NY TImes article was cheap shot, but deep down don't most of you think there is really something there? Jsut no proof at the moment.
So, all in all, I wouldn't bet more than $80-$85 that he would be the eventual nominee. You are also risking the health of the sportsbook 6 months down the road.