The easiest option is Аэрофлот: https://www.aeroflot.ru/cms/zh. One-way economy class flight direct from Moscow to Guangzhou costs about $250, or if you book 4 months in advance less than $200.

A year ago when I was working in China, I flew several times from Novosibirsk to Hefei and back with China Southern. It was the cheapest option with several stops but they always provided a hotel stay. I think they might have better options for Moscow. Try Hainana airlines as well.

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