President Donald Trump's eldest son and grandson took a hunting trip to Mongolia last year that reportedly cost US taxpayers over $75,000 (£59,000).
Most of the money went to their Secret Service protection, the group said.
Researchers say the Trump family take 12 times as many trips as the Obamas.
The Crew report accuses the Trump family of draining the Secret Services' finances with their average of 1,000 more trips per year, many for leisure, than the previous first family.
"This trip is just one example of the expenses the Trump family is incurring with American taxpayers, and if just one of Don Jr's trophy hunting trips cost more than $75,000, it's staggering to think how high the total bill must be," the report's authors conclude.
An employee of Mr Trump Jr told CNN that the eight-day trip last August was privately paid for, with the exception of his security detail.
Mr Trump Jr is permitted, but not required, to use the Secret Service for his protection...
According to a ProPublica investigation released late last year, Mr Trump Jr killed a rare Argali mountain sheep and only retroactively secured hunting permission from the Mongolian government.
The Argali sheep, famous for its long horns, is considered near-threatened, according to the Red List of Threatened Species...
Unlike his sister Ivanka Trump, Mr Trump Jr holds no official government position.