Exchange Stores in Airport or Border Ports
Such as HK Airport, LoWu Port
Worst converting rates
For example on 2017-03-28: You pay 100USD, get only approx 600-640CNY
     Usually they are operated by chain store company, in monopoly position, with a worest rates.
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Exchange Stores in HK downtown
Such as in Harbour City or streets everywhere
Very bad converting rates
For example on 2017-03-28: You pay 100USD, get only approx 610-660CNY
    The only good point, you can find it so easily. This kind of store may be operated by personal or cooperate chain. In HK, any person can apply for a licence easily for exchanging business from the Gov. In some bad cases, while you go to a store and inqury the rate, a clerk won't answer you, and just point to a info board on wall which shows a good rate. Once you give your money, they will show a contract to sign up. After you signed. They will give money which is much worse then you think. If you call polices, policemen will say they are unable to force the store to keep the rate on wall as you signed up the contract.
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Banks in Hong Kong
Such as Bank of China(BOC), HSBC, Citibank
Bad converting rates for small amount
For example on 2017-03-28: You pay 100USD, get only approx 645-670CNY
     BOC is a bit better than other HK or US or EU banks, as it is professional in RMB(CNY) business.
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Banks in mainland China
Such as BOC, ABC, ICBC in Shenzhen
Good converting rates, no min limit in amount
For example on 2017-03-28: You pay 100USD in cash, get approx 682-683CNY
     In mainland China, all banks can supply a good rate(except their branches inside airports). But only provide services in business days, and you have to wait at least 30 mins in bank for services. Most of banks do not have plenty of foreign currencies to exchange(BOC is better to exchange currencies, to get RMB are available in each bank.) Also, have a max limit of amount(5000USD).
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Exchange stores in mainland China
Such as Travelex, ICE in airports or downtown
Bad converting rates
For example on 2017-03-28: You pay 100USD in cash, get approx 630-675CNY
     In mainland China, there are exchange stores in airports or luxury hotels of mainland China. They are with bad rates. And have a max limit of amount for foreigner, according to law(5000CNY).
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General Speaking, Hong Kong is a capitalism market. High rent, High labor cost. Will generate a high cost in every field, including the currency exchange. Also, If exchanging GBP, EUR, JPY, CAD to CNY etc, the gap will be larger than USD to CNY.