Sack-Back Gown with a Half Skirt (robe à la française)

The width of the skirt of this gown indicates it was worn at a formal occasion. The fabric suggests this as well: the foliage pattern in the silk fabric is woven in gold and silver thread. This dress has a half skirt, a tablier, instead of a full one, which only filled the opening at the front of the sack-back gown. Could the reason for this have been because it was less expensive?