I am pretty sure the last issue is #375 titled The Wandering Starrs.
For a complete title listing check out the web-site Girls Comics of Yesterday.com. Excellent research!
It's really only the early issues that have any significant value. The first 20 or 30. You do see a lot of later numbers on eBay UK for as little as 99p. The value is in nostalgia rather than significant characters or artwork.
For a terrific gallery of covers check out http://library.doeth.net/judy/.
Note: some PR issues may have no value. Selected issues in FN, VF or VF/NM may not exist in some titles.