They weren't hard to pin down when the GAO issued a report on Presidential travel. In fact estimates seem to be on the low side.
You can read it here while you're waiting for your immunity to kick in.For the President’s four trips to Mar-a-Lago from February 3, 2017 to
March 5, 2017, we estimate that federal agencies incurred costs of about
$13.6 million.
https://www.gao.gov/assets/700/696512.pdf