The combine costs a lot more than $120k.
But farmers are not dumb. They have done these calculations. Monsanto seeds do not require greater long-term expense. It is no problem to quit using Monsanto's seeds if the costs are too high. You are not "locked in" to Monsanto's product. Many farmers can and do plant non-GMO seeds and if you've seen the prices of "organic" corn and soybeans you will know you can make money doing this.
If Monsanto's seeds quit being worth the money to plant, farmers would quit planting them.
It is not analogous to say buying MS products for your business. With those you do get locked-in when your Exchange server won't migrate out to a FOSS alternative. Or when your Oracle database system won't export to new products.
With farming though, all you have to do is buy different seeds next year.
Also, many people don't know this, FARMERS (by and large) DO NOT SAVE SEEDS.
Even the non GMO seeds are hybrids with enhanced vigor. If you saved the seeds the "hybrid-vigor" (google that) would not persist. This is not due to scheming by seed companies, this is just how genetics works.
Some people get so into the idea that Monsanto is evil that they forget to think if they are actually making a good product.