Cost wouldn't matter. The idea is that you can't use the company's resources for your personal projects, especially if that project is being used against them. So let's say you want to put up a single flyer about unionizing. You can do that in the appropriate area, such as off the company property or potentially on a bulletin board in the break room. What you can't do is type it up while on the clock and print it on the company's printer.

The other way (and it seems the NLRB sees it this way too) is that she didn't materially hurt google by doing so, and on the flipside used this system to make sure that laws are known and can be followed.

This is (a bit) like google buying property around polling stations and then preventing anyone from passing through to vote. They're not hurting google by passing through, but they keep shouting "get off my land!" Why would we let them weaponize property law to prevent others from enforcing their rights vis-a-vis google?

Did she make the change on company time? If so, her wages would be material damage. Even if made off the clock, it could still be considered material because Google needs to pay someone to fix it.

Your voting/property analogy is completely off-base. It completely ignores right of ways and public property. And if you have people actually trespassing on private property on the way to a polling place then they are in violation of the law and can be cited. I see no legal way that you can "weaponize" property law to prevent access to a polling location. Also, there's nothing stopping employees from exercising their right to concerted efforts off company time and property.

I think the far more substantial damage is from Google customers wondering what else the employee or some malicious insider could have done. You can bet that this came up in internal audits and customers' security assessments.