The Washington Post concludes that industry claims that state laws requiring food labels to disclose a products contains genetically modified ingredients would cost families $500 more in groceries each year are wildly exaggerated. The article is here.
As the article indicates, the costs claimed by industry are largely due to expected changes in consumer behavior. That is, if food labels disclose the presence of GMOs, consumers will opt for products without GMOs, which are purportedly more expensive. The argument that the cost would rise thus incorporates the recognition that consumers don't want GMO foods.
The $500 figure assumes that companies will switch to more expensive, non-genetically modified ingredients, and then pass all the incurred costs to consumers. It also assumes that all extra costs to stock, warehouse and produce new, non-genetically modified products will translate to higher prices at the cash register. It is difficult to imagine all of these assumptions will materialize for every company.