Try one packed lunch and compare the cost with your usual choice.
Quick action: 10 minutes · One-off test
Why it can help
Testing a change once is more useful than assuming you must do it every working day.
How to try it
Use food you already have where possible, then compare the approximate cost with what you would normally buy.
Things to consider
Convenience has value too. Keep the change only if it works for your routine and budget.