It is nearly impossible to find good data sources for agriculture if you do not know where to look. Below are a few places where you can find good data on row crop agriculture.
University Crop Budgets
Universities publish lots of research on agronomy and ag economics. With the goal of a profit model in mind the most salient publications will be crop production budgets.
These universities are good places to look:
U of Arkansas
LSU
Mississippi State
Texas A&M
Purdue
Cornell
Iowa State
Nebraska
Kansas State
All these Universities publish good stuff, but they have different regions and crops. Some have specialized areas like Nebraska having deep research on cost of running farm machinery and Cornell having very broad economic and market data.
https://www.uaex.edu/farm-ranch/economics-marketing/farm-planning/budgets/crop-budgets.aspx
http://www.agecon.msstate.edu/whatwedo/budgets.asp
USDA NASS QuickStats
https://quickstats.nass.usda.gov/
You can download a lot of different data from this site. For instance, you can find data on the average per acre soybean yield by county in the entire US.
This is the best place to get data on crop acres, production, cash prices for crops, land rents, inputs costs and general economics. The most detailed level is county.
There are usually methodology papers on the site. If not you can call their 1-800 number and get help finding it.
USDA NASS Ag Census
https://www.agcensus.usda.gov/Publications/
The title explains it all.
USDA ERS
Data Products – https://www.ers.usda.gov/data-products/
State Factsheets (good for general info) – https://www.ers.usda.gov/data-products/state-fact-sheets/
Crop Prices
https://www.quandl.com/ – my favorite for historical futures prices
DTN – the best place to get historical basis and cash prices for grains across the country
Pay Dirt 