How would this work though? You're not ordering your food via the QR code link, you're telling the waitstaff. Unless they ask you what price your saw, how are they going to correlate their variable price to a particular customer?
However, this would make it a lot easier to implement "peak pricing". Their menu could automatically update based on time of day, or day of week, and certainly holidays.
Can you imagine living in a world where American farm subsidies went to make fresh fruit and produce affordable to all income levels, but the fuel lobby had to pay top dollar if they want to distill corn into ethanol so they can dope our gasoline?