Hi Sharon. Are you looking to buy or are you the owner of the unpercable land? If you are the owner, I would want to know how long ago the most recent perc test was done. As John mentioned, there may be alternatives available of which you are not aware. I am in CA and I know that there are a lot of atlernative septic systems available that were not available 5 years ago. While the systems are available, they are not all approved everywhere. That's why you have to check with the county where the land is located.
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.