Come on man. First of all, these laws are almost always enacted with a time frame for implementation. Therefore, the new labels would only be placed on subsequent cans or packaging. So no reprinting would be necessary. If it did cost an extra penny a can - which it wouldn't come close to - that would mean an increase of 3 cents a can with standard mark-up. There is exactly ZERO extra time involved in printing labels with a little more information on them. Just a one time cost in changing the label itself.
I'm very familiar with the labeling process and its requirements. Not only am I a producer of labeled products, but I am currently working on a deal that could put my products in some major stores (stay tuned y'all). How in the heck do you figure a printer is going to have to run any longer or that workers are going to have to work longer in this process? Do you think they are using some dot matrix printer that scrolls along the paper and guys are sitting there waiting to peal the label off the printer? The modern printer has absolutely no change in it's process no matter how many words or graphics are on the label. An increased cost of printing the labels is
literally non-existent. They spend real money virtually every year to redesign the graphics on their labels and THAT doesn't cost them a penny a piece. Changing the marketing and such - sure. But there is nothing about the ingredients list or nutritional information that would affect that.
This is probably a more likely example of the costs involved. Lobbying - maybe the biggest cost. The lobbyists would shift to a new battle so don't think for a second that this "expense" would go away. Legal - the company's legal department would have to work out the wording to coincide with the new law. This wording would be used on most products Del Monte (for example) produces with only minor changes - it's known as boiler plate wording. And in reality, the implementation language of the law would give them parameters of what the labels could say or had to include. Any bill or law carries implementation language if passed. So the legal work there would be negligible. Look into the point of origin laws and you will see it is pretty simple.
That is about it. The companies already know what is in their products. They would just have to say. What they don't want is for cottage industries to get a stronghold or even a place at the table in their market. Sure that might cost them. I get that. But letting the industry dictate what they tell the public would have resulted in no ingredient labeling at all. These same companies have fought against all those laws except in the rare case that it offered them a market advantage.