In any case, I wish you good luck in your challenge!
A Broker's Price Opinion and appraisal could help however I am assuming that since you know that there are no solds on lots in your area that you might have already spoken to a real estate agent. An appraiser might have the same problem and have to go out of the area. As you said, if you go into more upperscale areas that the values are higher.
You might want to see if there are any homes in your area that have sold and then look at their tax assessment. The assessment is usually broken down with the land separate from improvements (house) and see how that was assessed.
If you contact an appraiser, he would check to see if there were any private sales and not just those through a multiple list system.You might not be getting all the information to help you with appealing the assessment so it would be in your interest to contact a professional that is knowledgeable of your area
. Good Luck!