I'm not a local agent in your area (I'm in San Antonio), but an automated valuation is the best you can get on the Internet and you can get this on Zillow. For a REAL price opinion, an agent will need to pull comparables on acreage prices and visit the location to identify pros & cons based on the comparables. Asking a reputable agent to value your property via e-mail is like asking a doctor to diagnose your medical conditions via e-mail only.
If you find anyone that will even attempt a site unseen valuation, I would caution you on entering into a listing agreement or the validity of the value identified. Good Luck!