I know for a fact that korean athletes get paid a lifetime pension to supplement their income should they win a Gold medal. It translates to roughly 3k US/month that's tax free with a limit of up to to 2 pension (if you win 2 gold medals). So yeah you won't be able to make a "good" living off of a single gold medal but it also means you won't be needing to work at a home depot for min wage to pay for rent either.
A lot of these athletes take on min salary jobs (which combined with their tax free pension money) from universities and train the next generation of athletes. In other words they get to keep training their sport long after their spot at the Olympics by having that supplementary guaranteed income from the government.
It's not what the Olympics is suppose to represent given that the Silver and Bronze medalist will get the shaft (compared to the gold), but it's a very effective and fair program that insures athletes aim for the gold (and why S korea with such a tiny pop still does decent at the olympics) and takes care of their future athletes.
PS: As for Phelps, for every superstar Olympians who receive massive financial support from companies, there's a gazillion out there who won't get shit. This is why I feel Canada at least needs to pony up the money to their athletes and institute some kind of financial support after their prime. A nice chunk of cash is good but it'll eventually go away through living above their means or bad investments. A pension even a small one will help them keep on doing what they do best.