Well, some of it is well known, he has some very rich UK backers who made significant donations to UKIP. What's still a bit murky is where the money that Aaron Banks gave to the leave campaign came from. There are indications that some of it might have been illegitimate and possibly from Russia, but so far there is no definite proof.
We also know UKIP was under investigation because they possibly misused EU money for the anti-EU campaign (which would be very cynical in the first place!), but I have not heard the final result of that investigation.
I assume he's handsomely paid for his various media engagements from the likes of Fox News, LBC and possibly more (not sure if he does anything for RT?). There was a report somewhere recently that the company where he channels his media income (probably avoiding a bit of tax on the side...) has increased its assets by about half a million over the last year (assumed to be mostly if not all income from his media activities).
From what I've seen on Twitter and other sources the march seems to be a bit of a damp squib (certainly so far) with only about 50 (if that) marchers taking part. I wonder what the chances are that it will be abandoned before reaching London?