Treasury Secretary Steven Mnuchin is under investigation for unethical use of taxpayer funds for travel expenses. Last month, Mnuchin used an Air Force jet to fly from Manhattan to Washington, D.C., an hour-long flight that cost $25,000. Earlier in August, he and his wife used a military jet for a "business trip" to Kentucky. Also in August, Mnuchin requested — and was denied — the use of a military jet for his European honeymoon. Mnuchin is reportedly worth $300 million.