I would say that most Fiction House good girl art covers (in FN to VF+ condition) go for around guide or slightly higher. Lower grade copies of non-classic covers can be had at slightly lower than guide prices. There are some books like Wings Comics #89-91, 94 that are extremely undervalued in the guide, and tyoically sell for multiples.
Let's put it this way, I have over 200 GGA books in my collection (Fox, Fiction House, Nedor, etc.), and on the average I've paid about 1.4 X Overstreet value. Of course, some of that cost includes slabbing fees and overgraded books.