(Mick Mulvaney.REUTERS/Carlos Barria)

White House budget director Mick Mulvaney issued a reminder for administration officials thinking about flying on private jets conduct government business.

Mick Mulvaney sent a letter on Friday telling officials to think twice before rounding up private air travel on the taxpayer's dime: "Government-owned, rented, leased, or chartered aircraft should not be used for travel by Government employees, except with specific justification," the letter from Mulvaney read, citing existing guidelines that have been on the books since the early 1990s.

Mulvaney said that beyond those rules, the heads of all executive departments and agencies should remember that the money they spend comes out of US taxpayers' pockets. "We thus owe it to the taxpayer to work as hard managing that money wisely as the taxpayer must do to earn it in the first place," Mulvaney's letter read.

The letter comes after health secretary Tom Price resigned on Friday amid a growing scandal over his use of private and military planes, on which he traveled for government business. The roughly two-dozen domestic private flights Price reportedly took in the last few months have cost US taxpayers $400,000, according to Politico, which first broke the news of Price's travel habits.

Sen. Chuck Grassley, chairman of the Senate Judiciary Committee, echoed Mulvaney's warnings to be mindful of taxpayer dollars in his own statement Friday, but the Iowa congressman went a step further: "It's not just the cost of flights," Grassley said.

"It's also whether attending a conference or a meeting overseas and planning a vacation around those events is in the public's best interest." That was an apparent reference to US Treasury Secretary Steve Mnuchin who watched the solar eclipse at Fort Knox while traveling in Kentucky with Mitch McConnell in August. Since then, other Trump administration officials have been scrutinized for their air-travel habits as well.

President Donald Trump indicated this week that he was "not happy" about the "optics" of Price's private private-plane use. Price had said he would reimburse the government about $52,000 for his seat on the private domestic flights.

Price's demise apparently had more to do with his overall record in President Donald Trump's administration. The flight scandal was just the last straw, according to a new report from Politico published Friday night.

The health secretary's personal stock within the administration had waned significantly in recent months following repeated failed congressional attempts to repeal and replace Obamacare, a signature Trump campaign proposal that so far remains unfulfilled nearly one year into Trump's first term.