Early issues are rare! A first issue with the back cover missing surfaced on eBay UK in Sep 2011.
The most sought after of the Girls' Picture Libraries but again the value is really only in the early issues and for the nostalgia of it all rather than sigificant characters or artwork. There are web-sites dedicated to these titles like Girls Comics From Yesterday, well worth checking out.
The 'big four' titles are Bunty, Debbie, Judy and Mandy Picture Story Library.
Note: some PR issues may have no value. Selected issues in FN, VF or VF/NM may not exist in some titles.