This is one of the difficulties of using public websites to research property values. I'm not sure how the website is calculating these averages, but I suspect that since the last sale of this property was unimproved (they bought the lot, then built a home on it) the comps may also be vacant lots.
You can view the listing for this property at http://realty-smokies.idxmedia.net/featured.php?listnum=1374 . It's a beautiful home in an upscale neighborhood, and is listed for $245,000. An offer of $50K would not be appropriate in this case.