She was only available in the first releases of Wave 2 so she is harder to find than non bang version as the latter got a wider release.
However I think she is only a little more sought after than normal Cheerilee. The significantly above retail price in G4 seems to be for 1) exclusives and 2) ponies which are not/have not had a re release.
Cheerilee has already been re released [all be it in different poses] several times.
Honestly I would put her $2-3 above retail as she is not going to be available in stores again.
You should go back to that store and see if they have the other pony from that wave - Lily Blossom seems to be selling about $15 at the moment