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It's actually not that hard to prove. For example, PSN now has dynamic pricing for their games which can vary quite wildly and all it takes is a small number of consumers with price differences to prove it. The same is true for grocery stores or whatever else.

Enforcing it is another question though and you're right that companies will likely just accept the fine. It's all the more reason why this sort of thing needs to aggressively be legislated against and denied.

If you can do traffic interception, there's a pretty good chance there's going to be traces of price levels in the API and the analytics. Especially since it's probably going to be bolted on to the side of anything PCI compliant. If there isn't, then it's probably going to be really easy to subpoena to prove mens rea, because getting that right is tricky and requires a fair amount of review and coordination.

Subpoenas and whistleblowing are pretty good tools.