If you have enough animals to supply even quality and quantity throughout the year then you have some negotiating power to request a bonus above schedule payment. A big farmer like Landcorp can do this. A stock agent can do this too if he has enough contacts with enough of the correct quality and quantity of animals.
I don't use a stock agent because I like the co-operative theory, and being able to get the extremely rare mistakes fixed by communicating directly with the company. When we had problems with our goats going to Grace our agents considered our problem too small to be worked on. We were paid 25% less than we should have been, but that was ... only... $400. When we eventually did find the person at Grace who could fix the payment problem, it took hum 3 attempts before he got it right . And we learnt lots of things that has stopped us breeding goats.
The payment to the farmers by the processors is most definitely a guess, and sometimes they make a profit and other times make a loss. But that is the same in every business, as Kodak found, and the video industry.
But .... I am going to become a millionaire by growing geese for pen quills , and sheep for wool.