In my area, if you sold it directly to another individual I'd say $350 would be about right. Maybe even as high as $400. But that's my area. If you try to sell it to a Pawn Shop, I wouldn't go under $250.
Personally, I would not sell a nice gun like that. If you are set on that course of action though, before you just sell it to a stranger, do see if other family members might want it. It is a family heirloom after all, and there may be a newphew or cousin someplace that would genuinely treasure that fine S&W. Plenty of stories have been shared here on THR about family members selling off granddad's Luger he brought back from Europe to the consternation of close relatives.
BTW, welcome to THR.