You should be able to date it from the componants used. Look here for you derailleur model:
http://www.disraeligears.co.uk/Site/Cyclo_Gear_Company_derailleurs.html. It looks like a sachs huret, which would date it 1980 or later, which probably means it is a later addition. Some componants are also stamped with manufacurers code dates. Go by the oldest bits and try and find two with close manufacture dates (hubs often have this)
Once you have a rough idea of the year(s) have a look at the catalogues here:
http://bulgier.net/pics/bike/ There is a 1975 CB cat with colour and frame details. (The blue under your gold paint looks quite like the blue they are describing as a "new colour" in the 76 cat., although by then all the bikes have cotterless chain sets, so yours would appear to be earlier than 1976.
What its worth sepends on how well you restore it and what anyone wants to pay on the day.
CBs were normally always made with good lightweight tube, I wouldn't get too excited about 531, but the catalogues shoiuld help here too. As has been said, weight of the frame is a good guide.
Happy hunting!