The easiest solutions are to either avoid the money-raising aspect completely (corporation puts up a big pot and the task winner takes it) or have the deck stacked for them ('Here are two ballrooms, each one filled with an equal income level of donors. Good luck.'). A creative production department would be able to find dodges.
However, we're talking about the Apprentice crew, so you've got a really legitimate point.
As far as getting to the end is concerned, I don't think we'll see another major black book fundraiser before the finale: as you noted, people are tapped. We're probably looking at more corporate prizes with a slight chance of Donald-cash (presuming he still has any) until we get down to F2. But once the curtain goes up on the rigging of your choice, we probably are dealing with check extractions again. For the existing donor pools...
Maria can presumably reach out to more wrestlers, plus if she hits the 'Oh, what the hell' mark, she can dig all the way back in her reality history and see if anyone at TBS is taking her calls. But it's a narrow field, and not all WWE employees pull in high salaries.
The joke of Holly's mega-check is that just about two-thirds of it might have been money she would have gotten anyway in the course of a Donald-free year. Mark her gun as empty: it takes at least six months for the next shipment of bullets to come in. Unless some of her former costars remember who she is...
As a celebrity chef, Curtis has other celebrities chefs (maybe... it's a competitive field), people he's cooked for, and maybe a Food Network executive or three. Potentially a major pool if he's arranged some big parties, potentially a tiny one if all he's done is publish a book or two. Big question mark -- but figure him for the low end.
Cindi may not remember half the people she knows.
Bret knows a very large number of people, very few of whom have any money. But if you ever need several truckloads of used panties, he's your man.
The editing gave off the impression that Sharon didn't manage to contact everyone before the deadline hit: she may have a donor or two lying in reserve. But that's potentially it -- one or two.
And Summer can presumably still get a giant bucket of slime delivered on command. (The IOC omly takes money.)
Who's got the most left? On raw potential, the highest ceiling might be Curtis -- but he's also got the lowest floor, and no way to tell where the elevator is resting. But if each remaining celebrity is allowed to assemble a team of the fired, then it comes down to which of the departed has contacts remaining.
I'm not sure it ultimately matters, though. Last year pretty much proved that total cash raised wasn't Donald's determining factor for a final winner. At least, not so much as the final name on the last revised script.