I don't have a blue book, but from what I've seen I would say $350.00 on the low end to $450.00 on the high end in the condition you have described. I had a 1974 Cobra (lightweight Detective Special) in like new condition that I paid $350.00 for and later sold for $400.00.
Last mfg. suggested retail was $429. And in 100% condition $425 in book. The model you mention ran from 1973-1986. As far as DAs the Python is the king of the hill. And will always retain value. The Detective Special is almost a Saturday night special. The horse saving it from this list often.