There is an idom bang goes something.
Bang goes something means - something has turned for the worse, something's gone.
If I don't get my knee looked at and fixed right away, bang goes my swimming for this summer! (= I can kiss my swimming good bye, there goes my swimming!).
James McLaren invested 50 grand in venture and bang went sixpence.
I am not sure if it's just a play on words based on that particular idiom.
"Bang" in slang also means luck/situation/ fortune
sixpense probably stands for something worthless.
paraphrasing this I get the same thing:
his luck/fortune turned sour (worthless) by investing all that money.