A person familiar with Mueller's investigation told the news agency Reuters on Tuesday that Germany's largest bank received a subpoena from the United States special counsel several weeks ago to provide information on certain money and credit transactions, confirming a report by German daily Handelsblatt published on the same day.
Mueller's office says in the report that about $1.7 million was for salary and benefits, and more than $223,000 was spent on travel-related expenses.
As special counsel Mr Mueller, who headed the Federal Bureau of Investigation for more than a decade, has the powers to subpoena records and bring criminal charges.
That money has helped fund 17 attorneys working on the probe, as well as Federal Bureau of Investigation agents, support staff, travel, rent, acquisitions of equipment and other expenses.
For example, Patrick Fitzgerald was a special counsel who investigated a far narrower subject: a leak that exposed the identity of covert Central Intelligence Agency officer Valerie Plame.
Judicial Watch, a conservative watchdog, last month sued the Justice Department in US District Court in DC for a copy of the budget.
"The cost of the Mueller investigation is far more than dollars and cents", he told Reuters in an interview.
Sen. Dianne Feinstein, a California Democrat and the ranking member of the Senate Judiciary Committee, called Mueller's spending "entirely reasonable given the results we've already seen".
Between outright denials of alleged collusion, open criticism of the FBI's reputation and the recent confession of his former national security adviser, Trump only heightened suspicions around him, especially with regard to his relations with Russian businessmen.
Mueller is investigating whether the Trump campaign helped Russian Federation in the Kremlin's efforts to interfere in the election to throw it to Trump.