There are a lot of variables to consider that we really can't answer without pictures. Just having certain period correct grips could increase the price estimate by $300-$400. Assuming it's not a registered magnum, and assuming it doesn't sport "coke bottle" grips, a great condition 357 Magnum N-Frame (pre-27), without box & docs, with a 6" barrel -- $800 - $1000 is about as close of a guess as I would care to take based on the info provided.