She is one of my wants right now! =) Shes less common to find as a loose single because there are so many re-releases of Sweetie Belle without the forelock. She was a 2010 release with Pinkie in the Sweets Boutique, then in 2011 they gave her no forelock, then in 2011 later they gave her the no forelock with the updated hair pattern. I dont think forelock Sweetie Belle is very expensive, shes probably around $5-$12 loose. I have some of the mane 6 with forelocks and I bought them on here and ebay as loose singles, ranging around $6-$8 loose so forelock Sweetie Belle cant be that expensive! If anyone has her for sale lemme know! ^-^
As far as the other fillies, Star Dreams Id say she is the rarest filly because she is an international exclusive. Ren-Ren recently had her in a group order cheap, making her easy to obtain! =) The unreleased Candy Filly.... well she is just a prototype for now but I consider her kind of rare right now too. I am trying to find the Apple Bloom that came in the Around the Town Train Playset! She is hard to find as well but not worth much. She looks almost just like the updated posed leg Apple Bloom but she has the older dull-maroonish hair. She is kind of pain to find loose because of the playset she is in. =(