It means to completely include the cost that comes with a risk. When you invest money, you balance risk (the chance of losing your money) and return (the amount of money you make). The writer is saying that investing in China is riskier than people realise.
"on such fine margins"
"Fine" in this phrase means something like "thin". "Margin" means the boundary between two things. "Margin" also has a lot of meanings in business and economics. As a whole, the phrase means that costs are very close to profits - if profits decrease or costs increase, companies will fail.
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