Here's the thing....
I can say with certainty that after spending tens of thousands if not hundreds of thousands of dollars and pouring hours upon hours into mind bending work, you will NOT be satisfied coming out of school and working evenings and weekends at America's Best seeing 6-8 patients an hour for $90000 a year with a high school graduate for a supervisor.
You may not want the hassles and aggrivations of running a practice. That's fine, though that is still where the real money in this industry is.
You might THINK you will be content at America's Best because most people entering graduate school have not had jobs beyond waitressing, college work study and working at Hot Topic at the mall so the idea of $90000 sounds pretty good but that fades incredibly quickly.
As far as $250,000 at Walmart, I would be interested in knowing how anyone knows that. Did they tell you that? Did you see their tax returns? Everyone can bluster and blather about their income. Most people lie through their teeth. Also, that person can be let go by Walmart at any time for any reason with a simple 30 days notice.
To make that kind of money in a Walmart environment almost universally requires a doctor to have multiple leases and to employ an army of worker bee ODs for $400 per day. If you're the lease holding doctor, lucky you. Those situations are few and far between. Most Walmart ODs are of the worker bee OD variety. If you're ok with that, have at it.