My guess is that the big opportunity which is generating the funding for Farage is for US health insurance, hospital and agriculture businesses to get access to the UK market as part of the trade deal with the US which is bound to follow any hard exit from the EU. Banks is in the insurance industry and I doubt he'd be funding Farage's lifestyle out of the goodness of his heart.
There is business logic for US billionaire republican donors putting a few million in to help dicks like Farage and its not a coincidence that Farage has said he thinks the NHS needs privatised. He was talking to his money, not his voters.
Farage doesn't have to win to be worth the money, he just needs to force the Tories towards a hard Brexit and comprehensive trade deal with the US.