You may be able to get an estimate of value by searching the county tax records [Marathon] and via the various websites such as Trulia, CyberHomes, Zillow, etc.
In the end, an independant appraisal will be the best thing to support your position. I would wait and see what you are being offered by the city/state/county. Who Knows?
Often, but not always- the offer is better than you could imagine.
WHAT IS EMINENT DOMAIN?
Eminent domain refers to the power possessed by the state over all property within the state, specifically its power to appropriate property for a public use. In some jurisdictions, the state delegates eminent domain power to certain public and private companies, typically utilities, such that they can bring eminent domain actions to run telephone, power, water, or gas lines. In most countries, including the United States under the Fifth Amendment to the Constitution, the owner of any appropriated land is entitled to reasonable compensation, usually defined as the fair market value of the property. Proceedings to take land under eminent domain are typically referred to as "condemnation" proceedings.
By Aaron Larson
Law Offices of Aaron Larson