I have someone visiting me in the United States from South America soon, and they offered to bring me some dry soybeans because they are much cheaper there. Would that be a problem? What are the rules about transporting food internationally?

They could transport dry soybeans; the problem arises with live plants through customs. The weight might be a factor in their luggage though, depending on how many soybeans they are bringing up.

I live in Canada and soybeans are not expensive here.

Dry Soyabeans won't create any problem dry things can be easily imported. Eatable are not allowed like some kind of snacks if you think you want to eat it on plane that is not possible. But Soyabeans are absolutely fine.