By far one of the highest salaries I heard for academics was around 350-400K. That was mentioned to us at an interview there by their director, just to sort of say that the faculty there don't have as much financial incentive to leave compared to others. Although there are plenty of other reasons to be happy or not happy, I can't argue with money being one of the bigger reasons for a lot of people. I think in general it is safe to say that academics pays far less than private practice. Around that place I listed above, the private practice people are still making a lot more. They work harder for it too though.
For private practice, I think a lot varies based on location and desirability. Also, I don't really know these numbers but just report what I have heard. In a good general job, I think you can get around 300-400 in a desired location like CA or HI, probably 450-500 in a nice location but not CA, and 600-800 or so in a less desired location - less populated town, midwest / central. ROUGH NUMBERS.
I was recently looking at academic jobs. The range I saw was from 160k to about 375k, the average around 250-275. As a general rule, more desirable cities pay less, and so-called more "prestigious" academic places pay less.
Also, the numbers quoted above by getinthehole for private practice are not for starter jobs or nonpartner salaries, they are salaries once you make partner.
No one starts as an Associate Professor. In some medical schools the starting title is Assistant Professor and in some medical schools it's Instructor. These were starting salaries. The increase in income in private practice is more than in academics, so the differential will become higher over time. In some places, starting academic salaries are on par with private practice salaries, but after a few years, the private guy will make much more since the rate of increase in academic salaries is much lower.