What's the difference between pre-galvanized and hot-dip galvanized steel?
They ate both hot dip galvanized. The difference is whether the material was galvanized before or after fabrication. Most boxes are made from sheet that has been "mill galvanized", which means the coil manufacturer galvanized the sheet on a continuous line. This coating is generally very thin (.5 - 1.5 mils) and during the fabrication by the box manufacturer, any cut edges will be left exposed. Also, any welds will damage the initial coating.
A box galvanized after fabrication will usually have at least twice the coating thickness, and any evidence of fabrication should have the benefit of being coated. And yes, is is usually much cheaper to build a box out of pre-galvanized steel.