Politico has an article currently with the juicy title: "GOP insiders alarmed by Trump;s fundraising." Of course they're alarmed, he's the first candidate who has ignored the big donors' priorities and campaigned on the base's issues.
The GOP base is accustomed to spending little or nothing on campaign expenses, letting the fat cats do all the heavy lifting. Naturally, the big donors who've funded GOP campaigns in the past aren't ponying up to fund Trump because he's not for amnesty, open borders, and globalization - issues they favor.
We've basically known we'd face this problem ever since Trump first burst on the scene and said "phooey" to the elites' agenda. Some of the big donors will come through, most probably will not. Either The Donald self-funds much of the campaign or us little folk will have to dip into hard earned and/or saved money for small individual contributions or he'll figure out how to campaign while spending significantly less, something he's done through the primaries.