As has been stated on here many times in one way or another, if farmland would pencil out everyone would want it.  I have never bought any land that would pay for itself in the short term.  Longer term is a different story.  

As with many investments it takes knowledge, experience and luck.  Not sure what the ratio is on each of these but luck does play a significant part in it.

Through the years I have been fortunate enough to purchase some land.  I would like to say it was my expertise and knowledge of the real estate market which made these farms good investments. Truth is I just periodically purchased a little land when it became available, seemed to be a reasonably good purchase and it fit in with the other land I owned. 

Through the years have paid anywhere from $200 per acre to $3,800 per acre for land.  Have been fortunate enough that the cheaper land helped pay for the more expensive....along with a public job.  

You are right in saying that the buying land does not make sense when it comes to the prices it commands.  Never has and never will.  The first land I ever purchased did not seem worth the money at the time.  A few years down the road it looked a whole lot better.  You just have to purchase it with the intention of hanging onto it for the long term and have the intestinal fortitude to shoulder the debt and not look back......and have a little luck. 

By the way, have also sold some land through the years when economic circumstances dictated it.  Things didn't always go as planned but that is what makes life interesting.