If you are not attached to it as a memory of your father it is worth more than most M&P Shields IMHO. Box itself in good condition maybe $200. The gun it self is worth over the $500 paid if it is mechanically sound. Just a shooter 45 colt, but a very beautiful one, Find any kind of big bore S&W for under $500 is rare. You could list it here in the for sale portion at a price you would like. If it doesn't sell after say a week drop the price and keep that up until it sells. You could put a link to this thread in the add and let interested parties review the opinions.

A dealer might trade you straight across for a shield, but they are in business for profit of course.