As I understand the big picture, Monsanto not only creates and sells the Genetically-Modified seeds (whose plants will survive/thrive in several times the amount of pesticides & herbicides that a normal plant can survive in *less weeds & bugs equals a bigger yield and profit for the grower*) - Monsanto also makes and sells the pesticides & herbicides that are recommended for the Genetically-Modified seeds they sell.
So it's not just the seed money they're after, it's the whole wallet!