I've had a few 80s Herman Marcus. Very work appropriate, nice, polyester dresses. One had a terrific novelty print.
Sizing is weird to generalize, because numbering has changed with vanity over the years. If it fits your daughter, compare to pieces in her closet that fit similarly - and then you have a somewhat accurate modern size equivalent. I always generalize a guess based on myself and my dress form, and give exact measurements by tape measure. The way a garment is cut, and the way it is supposed to fit sometimes are not reflected in the tag or in the tape measure - you have to take into account if there's an opening to get it on, and where it is supposed to fit snug.
As for value, we don't do that here. I can assure you this is a mass manufactured garment, and if your daughter loves it - she should wear it! I highly recommend developing a habit for wearing vintage - you will never be dressed the same as anyone else in the room, it is environmentally sound, and it is a wonderful way to express yourself, often very affordably.