AJ is about the only one who continues to hold an ok value. The repeat releases of all the others (whether identical or with minor changes) combined with the fact its clearly a style of release Hasbro are happy to keep releasing means the others have not held up too well.
Applejack - $50-60
Fluttershy - $20-22
Rarity - $15-18
Twilight Sparkle - $20-22
Pinkie Pie - retail (sometimes less as she is so prevalent)
Rainbow Dash - retail