Howdy
You can go on and on about how much it is worth, but the truth is, it is worth whatever the buyer and seller agree to. As far as the box is concerned, yeah, they are nice to have, but most dealers don't pay any more for the box, even if the gun comes with the box, all the tools and papers it came with new. They may try to charge more for the extra goodies, but they usually won't pay more. Same with the grips. The only guys who are paying that kind of money for grips are idiots. I see those grips all the time at shows for around $40-$50.
I just picked up a Model 28-2 very similar to yours in February. Mine has a 6" barrel and service grips, not the oversized target grips like your Dad's. Mine is not quite as pristine as yours, it has a line around the cylinder, like all Smiths do if they have been shot much, and a tiny bit of holster wear on the muzzle. Other than that, it is pristine. Mine was made in 1966. I paid about $400 for mine, if I recall correctly. I saw two or three others exactly like mine the same day, ranging up to around $500. I grabbed this one because it was the best deal. That's what 28s are going for in these parts, they are great guns, but they are not particularly rare.
By the way, what dash is your Dad's? It will be on the frame inside the cylinder crane. If you post the serial number I can tell you when it was made. You will have to remove the grips to see the SN, it is at the bottom of the butt frame. Don't post the entire SN, substitute XX for the last two digits.
I was just shooting my 28 on Saturday, and I love the gun.
By all means, keep it, don't trade it on something plastic.