NO ONE disputes that Secret Service protection is essential and expensive. But revelations about exorbitant rates that the Secret Service has been charged to protect President Trump at his private properties raise the question of what interest is being furthered. Is it the safety of the president or the bottom line of his private holdings?

An investigation by The Post’s David A. Fahrenthold, Jonathan O’Connell, Carol D. Leonnig and Josh Dawsey found 103 payments from January 2017 to April 2018 from the Secret Service to Trump companies, totaling more than $471,000. Among the charges were $650 per night for agents to use rooms “dozens” of times in 2017 at Mr. Trump’s Mar-a-Lago Club in Florida and $17,000 a month to rent a three-bedroom cottage at Trump National Golf Club Bedminister in New Jersey for three months in 2017.

The full extent of what the Trump Organization is charging the government is not known because the Secret Service has not listed them in public databases, even though it is typically required for charges in excess of $10,000. Nor has the agency filed the required twice-yearly reports on its spending to Congress. Since 2016, it has filed just two reports and those were not complete; the lines for Bedminster and Mar-a-Lago were both left blank. Post reporters were able to provide a glimpse into the arrangements by compiling documents that came out piecemeal from public records requests of other news organizations and watchdog groups and by talking to people who have seen some receipts.

Give credit to these reporters’ skills, but how public money is being spent is information the public, not to mention Congress, has a right to know. It shouldn’t have to be ferreted out. That would be the case even if the Trump Organization weren’t involved. But the fact that Mr. Trump stands to profit — unlike previous presidents who neither held on to for-profit businesses nor charged the Secret Service to use their properties — it is even more important that the information not be hidden. It is ridiculous that neither the Secret Service nor the Trump Organization will answer straightforward questions about the arrangements. And when answers are provided, they turn out to be false. Witness the assurance last year from Mr. Trump’s son Eric that “If my father travels, they stay at our properties for free — meaning, like, cost for housekeeping.” Housekeeper wages at Mar-a-Lago, according to a tweet from Mr. Fahrenthold, start at $11.13 an hour. Good then that the House Oversight Commitee last week asked the Secret Service to provide a full accounting of its payments to Trump properties.

Mr. Trump would like Americans to think that being president has cost him money, boasting how he has forgone his salary by donating it back to the government. At $650 a pop for a room and a captive audience, he can well afford to do so.

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