Why would labeling GMO status cost anymore than labeling sodium content? Just ad an extra line.
No. It's way more than changing the label. Food companies have an entire infrastructure to control the ingredients in their products. They have no infrastructure to decide whether they use non-GMO or GMO corn in their products. They would need to create that from scratch.
Farmers have no infrastructure to separate GMO and non-GMO crops. They would need to do that or decide to exclusively grow GMO or non-GMO, even if that is otherwise a bad decision. That would cost a ton of money.
Grain elevators would have to completely change their business and create a system of GMO and non-GMO grain elevators.
And, everywhere else in the supply chain would have to adapt to that. It's not a small change.