he doesn't have a great record of paying his creditors back

The Koch brothers have vowed to support down ticket races and hate Trump, but do they hate Hillary more?
Plus there is a lot of other dark money out there that will boost Trump.

Money on something he will get no return on and he knows is a losing situation. The self funding thing is a good part of his attraction.

His voters seem to just acccept pretty much anything he decides to say or do.

Add to that the Koch network, Rove's Crossroads organization, Republicans as a whole will spend in excess of $2 billion in this cycle.

He's a big businessman after all -- yuge! -- and there's no profit in spending his own cash when there are lines of willing rubes... er, suckers... I mean wealthy donors... um, loyal voters, willing to help.

Is that money in the bank?

and no longer claim that he is self funded.

As it stands now he hasn't spent that much comparatively, the MSM gives him free wall to wall coverage.