I found in general the following usually applies $0 - $3000 : Poor to Average Condition, parts and trim missing and usually a fair amount of rust. $3000 - $6000 : Average to Good condition, some may have rust spots, generally the cars are complete and usually modified $6000 and up : Good to Excellent condition, concours/show winners. To get a perspective on the current pricing go to
http://www.tradingpost.com.au, search for "fc holden" and select "all publications". You will get a listing of FCs for sale all over Australia.
Good Luck Keep in mind these cars are over 40 years old now so a completely original car would be hard to find.