For people who aren't aware of this, the IRS is so underfunded right now that funding IRS enforcement has an ROI of between five and nine, that is, for every dollar we put into funding the IRS it would be able to claw back up to ten dollars in unpaid taxes. That's the kind of number that you usually only see on "the next Google" type unicorn startups. It would take eighty billion dollars, invested over ten years, to fund the IRS to the point where diminishing returns bring the ROI back down to reasonable levels; those eighty billion dollars would return almost 200 billion dollars in newly collected taxes. Almost all of that would come from high-wealth taxpayers, large corporations, and partnerships, which are currently very under-audited due to lack of resources - the audit rate is currently lower for high-wealth entities than it is for random poor people simply because auditing people with a lot of money is complicated. The current audit rate is currently dramatically below where it was twenty years ago.

Source: https://www.cbo.gov/publication/57444