What is all that farm land worth? It is a question farmers may ask themselves, and passers-by of these vast expanses of green space wonder as well. The truth is, depending on where you live — and scores of other considerations — it IS possible to get a pretty good estimate.
In Wisconsin, The Agricultural Assessment Guide for Wisconsin Property Owners
lays out what goes into valuing 
acreage. It is comprehensive, and also a free download.
Other States such as OH offer their own valuation methods.
For farmers across the USA, the United States Department of Agriculture (USDA) offers land value averages by-state, in its 2015 Land Values Summary. Liars can figure, and figures can lie, but for starters these documents and others like them are great tools.