The whole point of the agency is to increase tax revenue. If it costs more to run their operations than they're bringing in by catching tax evaders, then it's not worth it.
You do however need to account for the deterrence effect of their existence. Individual operations might not be cost effective, but the news stories they generate might be enough to encourage people who were flirting with the idea of tax evasion to remain honest.
Second, this has to be balanced with the taxpayer’s costs of being audited.
What is good and the purpose of tax enforcement is that the system runs as the law defines it and is “fair” in the sense that everybody has to obey the law, without exception.
Miss a few payments and you'll soon see how much money they're willing to throw at you in threats to drive the message home that your life is over if you don't resume paying, all for very measly amounts like $300. Taking someone to court even for a few minutes is going to cost more than the $300, taking you to prison is going to cost astronomically more than the $300.
Government needs to keep up the meme that taxes are something you owe purely for existing on their land, this meme of course is only aimed towards the working and middle classes, elites are allowed to skirt around it and pay back in other ways.
To the government, the idea that you could know say 5 working/middle class friends who just decided not to pay taxes and had no repercussions is far more dangerous than 1 billionaire not paying taxes. One of these situations is going to make you feel like an idiot for still paying, the other you'll just write off as that 1 person being crooked.
This is the center of gravity of the tax system, the people who are in aggregate both well off enough and numerous enough to act as cash cows. Plus being employed limits tax shelters.
So keeping the upper middle class taxpayers compliant should be the top priority of a competent IRS, irrespective of fairness. I don't know how focused on that they really are, but I would give them the benefit of the doubt without definitive evidence.