This question's come up a few times over the years.  Fairly or not, a boxed set without the box is considered "incomplete", and no estimated value given.  That doesn't mean it's worthless -- but you should assume some discount percentage of the value of a complete set (and probably a hefty percentage).  This is because these items are "collectibles" and are not being obtained for the information inside -- which is almost universally available for free on the Internet now anyways.  Think of what the value of one rare earring would be -- without its companion.  Probably far less than half a complete pair.

Typically the value of "pieces" of a set is only in their ability to complete or upgrade a set that someone already owns.

Foul

You can follow this auction to see a real example of a similar case:

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The books can and do sell on ebay by themselves. And of course people are often attracted to buy things outside of "sharp" commercial value, but for me that price would be a bit high. For many games (not just D&D), the game box is a cool thing to have as it is what people remember really well ... so it might be worth holding out for a set that includes the box but is maybe missing a book (unless they drop their price).

Edited to add that the books can often be bought individually if you are looking for a nice display piece (see below for a quick example, that is a best-offer listing, so maybe start with an offer of $55 or $60...

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Thanks again, all.  Yeah, I figured the price was likely too high (even at discounted 74-cents-on-the-dollar cdn bucks), but it wasn't clear to me how much of the value was wrapped up in the set as a complete set.

LB