It should go for somewhere between $300 and $400.

.38 Special J frames are largely considered more useful than collectible.

And, square butts are generally considered a little less useful than round butts (due to fewer after market grip options and the relative "difficulty" in concealing their slightly larger grip profile).

Thus, a round butt will usually bring more money and move quicker than a square butt. A round butt, as described, would bring $375-$400 without much trouble. A square butt, as described, might take a while to move at $325-$350.