It's pretty common at least in Texas. ProTax (not the company I use) is one of the biggest in the market. Texas isn't a low-tax state; it's in the middle, but with no income tax, a lot of its revenue comes from property taxes. This includes school taxes, which the state takes, redistributes some of, and then uses the rest of balance the general books. So there's a lot of money to be made in small reductions in property value.

My tax rate appears to be like $2 per $hundred in valuation. So, if the appraisal board says my house jumped like $60k in value this year, that's an extra $1200 I'd owe. But the lawyer uses comps (which they found for lots of clients) that show my value should have only changed by +$20k. They protest, spend maybe 30 minutes total on my case (I assume), and get my valuation fixed. The difference in $40k valuation turns into $800 saved on my taxes, so I pay them $400 for their time and I pocket the other $400 they saved me.

Some years the county tries to up my valuation by $150k or more. This year it was a lot more than that. They got it lowered some. Texas caps your valuation change for the purpose of taxes at 10%, so if the county says my house doubled in value in one year, the double value shows up on the books, but I only pay 10% more. Then next year I can pay a compounded 10%, etc, until I catch up. That's why it pays to protest every year. The past five years when they got my value down $50-150k each year means my starting point for the cap is lower.