For the most part, those cost differences are due to US trucking being expensive and international shipping relatively cheap rather than anything specific to the modular construction. The same material (actually more due to waste) has to be transported to the site if you use different methods of construction. The need to fit on the back of a truck imposes an upper limit on the smallest dimension of a room if they're shipped as full rooms and not flat packed wall units, of course, but that's unlikely to be an issue for student apartments in Cali.