Earlier quoted context omitted.
The easy solution for that is a stipend and receipts when you go over the cost for a few defined scenarios (for example, $60 to get to the airport, $20 a meal). If you are a cheap bastard, you make a few bucks. If it took three train trips and a taxi to get to the airport, keep the receipts.
The bureaucrats feared exactly the result of what you propse: That people would spend more than they eat and pocket the difference. The old honor method was to add up what you spent on food and claim it as "per diem". The maximum was far more than you actually needed most places (Note: There's a big difference in food costs between, for example, Geneva and Bangkok). The new method required receipts with your name on…
And yet, allowing exactly that is the easy, rational way to cap costs.