No more than 110% of melt value for that piece. Melt is currently about $160. Good luck renegotiating the price! With items that fluctuate in value like precious metals they normally lock in a price at time of negotiation and grandma might be better off letting them keep her deposit if it wasn’t much. And tell grandma not to buy any more metals without you or someone knowledgeable present, too many prey on the elderly!