(CNN) If Donald Trump were to honor his pledge to spend $100 million of his own money on his campaign, he would have to invest and spend $44 million at breakneck speed.

And even then, Republicans warn, it would not do much good.

Trump, the billionaire businessman whose outsider appeal was bolstered by his seeming ability and desire to self-fund his campaign, only gave $31,000 to his effort over the first three weeks of October. And on Friday, he wired another$10 million to his presidential bid, but declined when asked by Fox News to guarantee a $100 million total.

"I'm going to meet what I have to meet," he would only say.

As recently as Wednesday, the Republican presidential nominee was insisting he would cross the $100 million threshold by the end of the election, something GOP allies have urged him to do for months to resuscitate his stumbling campaign and augment their capabilities on the air and the ground.

But even if Trump cut the massive check to hit the magic number -- and that's an enormous if -- it may be too late. He is essentially left with two bad options, observers say. He could try in vain to hire last-minute organizers or beef up data capabilities, but those fixed operations take months to develop.

Or he could write a check to fund a last-minute advertising barrage and come closer to parity with Hillary Clinton and her associated super PACs on the air -- a likelier scenario, but one with tremendous diminishing returns given the cost and availability of advertising slots with less than two weeks to go.

"It's much harder to scale up the turnout stuff because a lot of that is human capital. Can you find more people?" said Mike DuHaime, who has guided general election budgets at the Republican National Committee and for John McCain. "Putting a lot into the ground game at this stage is smart, but it's hard to do."

Trump's campaign has only $16 million on hand, largely thanks to his poor performance with the party's most elite donors, who only loaded $11 million into the high-dollar Trump Victory fund in the first part of October.

Clinton, meanwhile, has $62 million as of October 20, and her super PAC Priorities USA has an additional $15 million.

For Trump, this adds up to trouble for a candidate trailing or neck-and-neck in the swing states he needs to win in order to capture the White House, such as Ohio, Florida, New Hampshire, North Carolina and Pennsylvania. Given that Trump's path to 270 electoral votes is much narrower than Clinton's, the money deficit could easily prove fatal.

Trump has long been under pressure, both from donors and Republican National Committee Chairman Reince Priebus, to write a sizable check to his campaign.

It's been a struggle to convince Trump to contribute more, though, and one source says there were a number of previous instances where Trump appeared poised to offer a cash influx but ultimately didn't write the check -- much to the frustration of his allies.

While Friday's contribution is a large sum, one fundraiser was expecting an even larger donation, in the range of $20 million. This person says it was becoming increasingly difficult to convince donors to give sizable contributions to the campaign, in part, because they pointed to the fact that Trump was barely funding his own campaign effort.

Small donors helping Trump

Yet Trump has been kept alive by online donors -- plus his willingness, no matter how elusive it seemed at points, to dig into his billions of net worth and independently finance a bid.

Trump in fundraising emails Thursday promised to triple-match his supporters' most recent contributions received through Monday -- and he notably did not include a $2 million cap on that match as he has on solicitations throughout the year.

"With Hillary Clinton dominating the airwaves thanks to her huge contributions from liberal elites, and all her media friends lying about me on TV all day, our final ad blitz needs to be huge," Trump wrote. "If you wait to contribute tomorrow, it will be too late, Friend. We need to buy more 30-second ad slots now."

And he repeatedly told CNN's Dana Bash on Wednesday that the $100 million total was still accurate, flatly dismissing any skepticism and even suggesting he could give "much more."

"I will have over $100 million in the campaign, and I'm prepared to go much more than that," Trump said.

Why didn't he give more? And spend it earlier?

Experienced Republican budgeters have for months stressed that cash that arrived too late in the campaign would have limited utility, and that guidance has been especially sharp for Trump given that he started the general election with no fundraising or data infrastructure.

GOP operatives have fretted since the summer about states where Trump can afford just a half-dozen aides in a swing-state; or where second-tier media markets in battlegrounds do not see any television advertising; or where the infrastructure for tracking early voting is dilapidated and under-funded.

The date long circles on Republicans' calendar: Labor Day, after which, they argued, Trump would begin to see diminishing returns to his money.

Trump did not begin television advertising until late into the summer -- ceding the airwaves to Clinton until the nominating conventions wrapped -- and still maintains a skeletal staff compared to the Clinton behemoth in Brooklyn.

Yet Trump has steadily donated $2 million a month to his campaign for virtually the entire race, smaller chunks than he gave to the campaign during the primary, when he was self-funding nearly entirely. And Republicans now stress that his ability to self fund -- widely celebrated and popular during the primary -- is now largely worthless with two weeks to go, if he even does so.

How to spend $44 million in 11 days?

Jim Bognet, a Republican strategist who bought ads for Mitt Romney in 2012 and has experience with self-funding candidates like David Dewhurst in Texas, said Trump could easily find a way to spend $44 million in a tiny timeframe -- but that money would be spent carelessly.

"If you don't care about price and efficiency, you could definitely spend tens of millions of incremental dollars in a presidential race in the last 10 days of campaign," he said. "In crowded swing state airwaves, it's hard to get incremental space on broadcast TV at this time in true battlegrounds."

Bognet had a few ideas for the Trump campaign. It could air television ads in second-tier states like Arizona or Utah -- red states where he's facing strong challenges from Clinton or independent candidate Evan McMullin -- or do massive national cable buys on pricey channels like ESPN. It could invest big in radio, a relatively available medium where Bognet recalls making reservations in 2012 just 24 hours before Election Day. Or Trump could pay for turnout calls in every particular language -- even up to on Election Day.

"The problem with all of this is it is super expensive," Bognet said. "So the closer you get to Election Day, the more crowded with other campaign and super PAC ads are, and often the harder it is to either get bang for your buck or placement."

Trump during the primaries touted his ability to win on the cheap, especially compared to candidates like Jeb Bush who went nowhere despite spending tens of millions of dollars. He seemingly lamented the political spending game Wednesday in the interview with Bash.

"In the old days, you'd get credit: If you would spend less money and have victory, that would be a good thing," Trump said. "Today, they want you to spend money."

Trump has also struggled to steer his donors to a viable super PAC behind his bid, like Clinton has with her Priorities USA. The National Rifle Association has been Trump's biggest defender on television, and a second PAC funded by the powerful Adelson and Ricketts families showed initial signs of progress earlier this fall, but neither has matched the Clinton juggernaut.

One new donor to Trump's super PACs: Silicon Valley titan Peter Thiel, who gave $1 million to the super PAC Make America Number One -- run by the powerful Mercer family -- after the filing deadline, according to a Thiel spokesman.

Many Republicans, at this point, don't expect the major injection of cash to happen. And in fact, a rich irony was thrust Thursday upon Trump, a candidate who has attacked Clinton for not investing more of her own money in his campaign.

It was Clinton -- with her $50,000 donation -- who was the biggest self-funder in October.