Donald Trump’s net worth has plunged to $3.7 billion, down $800 million since 2015, according to Forbes. The report on the Republican presidential nominee’s net worth comes amid several questions over Trump’s reluctance to release his income tax returns.

In a report Wednesday, Forbes cited the "softening" of the New York property market as the reason for the slump in his wealth. Of the 28 buildings that Forbes assessed, 18 declined in value, including his flagship Trump Tower on Manhattan's Fifth Avenue. The real estate mogul’s property at 40 Wall Street and Mar-a-Lago, his private beachfront club in Palm Beach, Florida, also lost value. However, seven of Trump’s properties, including San Francisco’s second-tallest building, increased in value, the Forbes reported.

At Monday's presidential debate, Trump said: "I have a tremendous income ... it's about time that this country had somebody running it that has an idea about money."

Trump has boasted about his capability of funding his own campaign for the White House run and not depending on major donors unlike his rivals.

According to Forbes, Trump has invested about $50 million of his own money so far in his presidential bid. The report also stated that the 70-year-old’s controversial remarks about Mexican immigrants during his campaign launch cost him about another $100 million in lost deals with NBCUniversal, Univision and Macy's, among others.

During the filing of his financial disclosure with the Federal Election Commission this year, Trump’s campaign said that his net worth is “in excess of ten billion dollars.” However, critics have accused him of routinely overstating his income.

Trump's refusal to disclose his tax return has fuelled speculation that his disposable cash is not as huge as he claims, or he may not be paying his fair share of taxes.

In his defense, Trump said, during the debate, that he was “smart” for not paying federal income taxes in some years. He later pledged that he will release his tax returns once the government completes an audit. 

Hillary Clinton suggested that Trump’s tax return may reveal "something really important, even terrible, that he's trying to hide.”

However, the disclosure of tax returns is unlikely to give a true sense of Trump's real wealth.

The Democratic presidential nominee's returns showed that she and her husband, former president Bill Clinton, had $10.75 million in income in 2015 and paid an effective federal tax rate of 34.2 percent. Trump’s running mate, Indiana Gov. Mike Pence, has also released 10 years of his tax returns.