If the land is truly not suitable for residential purposes, the value will depend on what other uses are possible and you should contact a local real estate professional to ask that question. The local agent should also be able to give you information regarding septic system alternatives. Good luck.
The link below shows lot and land properties in Myersville that are currently listed on Trulia. Perhaps you can find a comparable property that will give you some insight into your question.
Customer Service Representative