And the chances of anyone with an income of $15K getting audited are the same as higher incomes?
The IRS doesn't publish these statistics obviously, but it seems that realistically they spend their time going after the bigger fish.
Think of them as a business, trying to maximize their earnings. If they assign someone to spend 2 weeks auditing you, and he earns $2,000 for those 2 weeks, and gets $500 more in taxes out of you, they lost more than they gained.
If however they go after a millionaire and get $50,000 more in taxes out of him, they gain a lot!