Most likely, you are talking about fen coins issued during 1955-1979. There was no other coin issued by Bank of China during the same period. No fen coin was issued during 1965-1970 due to the cost of producing those fens was higher than the value of the coins. In addition, there is no production of 1 fen for 1960 and 1962, 2 fen for 1955, 1958, 1971-1973, and 5 fen for1958-1963, 1971-1973, 1975, 1977 and 1978. The 1979 5 fen, with a mintage of 10,000, was produced for 3-coins mint set and never in circulation. The mintages of those fen coins are unknown, but are very low, definitely less than 1 million, most likely in the range of 300,000-500,000. To my best knowledge, no book, in English or Chinese, or published literatures about the mintages of these coins is available. 1955, 1957 1 fen and 1979 5 fen are considered to be the key dates. Recently, 1955 1 fen NGCMS66 was ended $161 at eBay. IMO, because of their rarity, any fen coin dated between 1955 and 1979 with a grade higher than MS67 should be valued higher than most MS69 1 oz silver panda . The asking price for 1957 1 fen BU (MS63-65) inside China is about $260. The asking price for UNC (MS60-63) common date (1971, 1977, 1978 1 fen, 1976, 1978, 2 fen and 1974, 1976 5 fen) inside China is about $1-$3, while recent eBay auction for those common dates with NGCMS65-67 were in the range of $35-$75.
Here are two active listings of PCGS coins at eBay;
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=200497391990&ssPageName=STRK:MEWAX:IThttp://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=200497351940&ssPageName=STRK:MEWAX:IT
