I'm not a fan of QR code menus either, but printed menus are not necessarily cheap. The owner of one of my favourite restaurants told me that he couldn't raise prices on his menu to match inflation because the cost of re-printing all the menus with increased prices would eat up the difference. IIRC, the restaurant later shifted to a cheaper and simpler menu design.
If you're a restaurateur, do you have the .ai files your agency created, an Adobe Illustrator license, and know-how to get in there and change the prices? And then know where/how to deliver the result to get it printed? If so, you probably still have something better to do...
You probably pay an agency an hourly rate plus markup to get them updated, prepped and sent off to be reprinted.
Next time: negotiate fixed prices/timelines for small updates, own the files, and own the relationship with the printers.
Even if he didn't have to adjust prices due to inflation, surely restaurants adjust the items on their menus frequently. I have been to a restaurant that printed new menus daily because it changed daily. It's like 10 sheets of paper or cardstock. It's not that expensive.