The reason Velma is so expensive is that she only appears in one set - the Mystery Mansion released in 2015 and not in particularly great quantities and was also the high end set in the theme. Daphne was in this set too, but she was also in another mid-range set which helps keep her price more reasonable.

Personally I'm not a fan of the Lepin stuff, primarily because I don't like the way they infringe on copyright and basically steal other peoples intellectual property to profit from it without paying royalties.