Mr. Trump is to be feted at two fund-raisers in Manhattan this week, organized by Woody Johnson, the owner of the New York Jets, with one event featuring Gov. Chris Christie of New Jersey. But the ticket price for that event is only $500, a paltry sum for a presidential campaign, and only 260 people have signed up, according to a person involved in Mr. Trump’s fund-raising, who spoke on the condition of anonymity because the information was not intended for the public.

Charles Spies, a Republican election lawyer who advised the “super PAC” that supported Jeb Bush, said Mr. Trump would have to put in an enormous amount of his own money to jump-start his campaign and win over big donors. He suggested an appropriate figure would be $100 million to $200 million.

Mr. Spies said Mr. Trump should also forgive the loans he had made to his campaign, to reassure contributors that he would not use their money to repay himself. Mr. Trump has already raised eyebrows among party donors by spending freely to hold campaign events at properties he owns, and for the cost of flights on his private jet.

“For donors to invest in his campaign, he’s got to show that he’s investing in it also,” Mr. Spies said. “He’s got to have $500 million to run a bare-bones campaign, and that would mean getting outspent by Hillary Clinton and her allies, between two and three to one.”

Dwight Schar, a former finance chairman of the Republican National Committee, said Mr. Trump’s grim predicament came as little surprise. Mr. Trump never courted party donors during the primary season and accused them of seeking to buy influence in government, boasting that as a wealthy man he would be immune to their entreaties.

“I think Mr. Trump has got all the money, so he doesn’t need any financing,” said Mr. Schar, who said he was undecided about whether to back Mr. Trump or Mrs. Clinton. He added, “I think my mother used to say, ‘What you sow, you reap.’”