Things understood:
Fully delivering from a farm or cattle or other simple producer facility will result in business profits around 10% of the business purchase price.
Delivering more loads from a meat packing plant in the previous year (the year you see when hovering profitability) will increase the profits of that year. Delivering about 2 cars worth per year seemed to result in business profits around 10% of the business purchase price, more would be higher. This aligns with Maglev's comment.
(on the hardest difficulty) Demand does not matter. I supplied a town bakery (w/ extra supply trains) before supplying grain from my farm (demand was 2). I also did a save and stopped the extra supply trains and then delivered from my farm (demand was 5). Farm profit was 25k each time. I delivered 6 cars that had been stockpiled in both runs.
unclear:
I was unable to replicate the lumber mill described. I had 50k (25% of business purchase price) average return sending to 0 demand towns using 2 lumber camps as a source even with some congestion. Most likely the OP was not sending enough per year due to high congestion.
I don't understand relative profitability. With the relative profitability of 100 for grain farm I expected profits to be considerably better than cattle but it was not. Both were around 10% of business purchase price.
untested:
Economic cycles affect on businesses.