There are so many variables involved with agricultural land that you really need to have someone take a look at the plot. For example, soil quality, grade, dimensions, road access, drainage, etc...
If the land is zoned purely for agriculture and has no opportunity of ever changing zoning status to something that could be built on, then it could be fairly low value, perhaps even down to a few zloty / m2.
What changes the game completely is the opportunity, or potential opportunity, to change the zoning status to "budowlana", allowing you to develop the property. There is a great deal of speculation involved in that area, and your question becomes how long is a piece of string?. You could then be looking at 100's of zl/m2.
You need to be aware that there are a lot of investors looking to take advantage of the Babkas in the village by purchasing cheap agro plots and having them converted to developable land. There are "expeditors" who know the "system" and specialize in having land converted for a fee.
Take a look at these guys, maybe they can help: plotspy.pl/the-investment.htm