Where do East End / Gangster slang terms for numbers relating to money originate?

monkey = five hundred pounds (£500). Probably London slang from the early 1800s. Origin unknown. Like the 'pony' meaning £25, it is suggested by some that the association derives from Indian rupee banknotes featuring the animal.

The origin of ton supposedly comes from the fact that a ton is a measurement of 100 cubic feet of capacity (for storage, loading, etc).

That link above gives a good summary of what's known about British slang money words and expressions. I will just say that I'm "semi-Cockney" myself. Like most Brits, I know the value of a pony and a monkey, but I've seldom if ever heard them used except "facetiously". Ton, on the other hand, is relatively common (though not so common as the US import grand).