I'm going to have to question that statement. Not because I have any specific proof against it, but surely something similar has happened somewhere. Maybe Ron Paul 2012? Dollar for dollar arguably more wasteful, but certainly not as many donors. That's probably the most wasteful campaign spending of all time (SuperPAC included of course), but mostly taken out of the pockets of gullible wealthy establishment folk.
Of course in no way am I attempting to undersell the burning of millions of dollars of beer/pizza/tuition money that's gone on for months now by Sanders. Given any new funds would probably be going toward air time in CA, that's an even bigger waste of ad cash than usual when he'll need an absolutely impossible 70%+ of the vote there. $0 or $100M spent, it simply will make zero difference. You could literally pile up the money in a gigantic mountain of $27 wads of singles, light it aflame, and accomplish just as much.