Serious question: Are anti-material rifles commonly used by cartels? They seem like overkill when AR-15 style rifles are cheaper, more plentiful, easier to use, easier to conceal, and are probably more effective against people.

Yep. The Dilldozer and the Ass Blaster.

“Cost” implies that it is money he didn’t already owe. “IRS Audit of Trump Finds Former President Underpaid his Taxes by $100m” is a lot more accurate and frames the result correctly. He cheated the IRS and they want their money.