Yeah but remember, there will always be a limit to the price they can charge for the GMO - and that will be determined by the cost of the wild type and the productivity different between the two. Thus Mosanto will only be able to sell it if it is worthwile for the farmers! Also, patents do expire eventually.

I've heard this before, but haven't seen such points respond to the (to me) super obvious point that IP protected GMOs have plenty of competition (natural crops) so they can only generate profits on their improvement. One response I can think of is the optimal length of IP rights varies from field to field and is not necessarily long (link). Do you have a link to intelligent discussion of such issues?

I get the impression this objection is raised mainly because people react negatively to ownership over things which are not typically owned rather than a specific technical argument for why it's a bad idea. This has lead me not to pay much attention to such arguments, but I'd be interested to hear more detailed arguments if you think there is more substance there.

The claim that "the purpose of GMOs is so Monsanto can have a plant they own.." is false. One tiny aspect of GMOs which objectors constantly harp on is Monsanto's business model.