Whatever someone who's job and education is to develop such policies says. It's not my profession and it doesn't have to be.
The point is simply that even merely picking 1% or 0.1% of people completely at random to audit keeps 99% of normal people in line, which is far more valuable to society (not just in immediate dollars) than the cost of those few actual audits, regardless what those audits "earn" in collecting a few, or zero, or indeed negative dollars that might have gone uncollected from a random individual. There is no reason an audit should not show that there was an error and the government owes the taxayer, let alone collecting nothing or collecting less than the cost of the audit.
The police's job is not to recover ypur stolen lawnmower, it's to maintain order in general. They expend many thousands of dollars in resources to track down a lawnmower theif not to recover your $400 possession, but to inhibit the activity of theft in general.
Tax audits are, or should be imo, like that.
The actual details of what should be written in the IRS manual are this: Something.
It's a meaningless question since we're not at that level. I'm only talking about the fallacy of treating tax audits as nothing more than a direct and immediate source of income instead of a means to maintain order and a far greater but indirect source of income.