Figuring Property Value

[Terrica Carlock]

While doing some research on a home and attempting to assess it market value, I have run into a little problem.  The home was built in the 50's and there are no comparables in the area.  Other than looking at the home in person and viewing the county appraisers estimates, can anyone suggest any other ways to assess?  Is there a "rule of thumb" when running into situations like this?