There are a lot of variables when it comes to lot pricing. For example, is it a lot with public water, sewer and natural gas, or will it require a hookup? Is it part of a development, or is it just open space. Also, what type or shape of the lot? Let's not forget the most important question in real estate, location, location, location (what school district is it in will dictate the taxes you pay). Are there any outbuildings? The answers to these questions can make quite a significant difference in the price of the lot.
The best place to start is to begin a dialogue with your neighbor or to get a realtor.