Generally yes, gravid females will cost more than non-gravid.
1. Being that they have eggs inside them that if fertile will turn into more snakes for you.Your essentially paying for her and her 'possible' babies.
2. If she was bred to a morph(s) and produces fertile eggs that again is more babies for you and likely an instant return on your investment depending on what she was bred to.
Only way that could go bad is if the gravid female lays all slugs/infertiles which can't really be blamed on either party,or if all the eggs hatch out as plain normals, but least you's get the experience of hatching eggs if you haven't already.