I have worked for companies that give a fixed perdiem and companies that will allow you to spend up to a certain amount on the corporate card.
Fixed perdiem rates go along with the IRS standards for meals and incidentals. If you travel for a FULL day to a high cost area, you will get $65, and for a low cost area, you will get $45. You will get that amount regardless of what you actually spend. Here, you are incentivized to spend as little as possible because you can pocket the money that you didn't spend. Because of my hotel frequent guest program, I almost always get a free breakfast and will typically take a piece of fruit for my morning snack or I will make myself a peanut butter and jelly sandwich from the continental breakfast. Then, I would go out for a nice dinner and spend $30-50. Some locations it's not enough to eat properly and you have to spend out of pocket, but other days you can keep most of the money allocated to you.
I used to work for a company that would allow you to spend UP TO $85 for high cost areas and $75 for low cost areas. It was basically a 'use it or lose it' system. In this system, you are incentivized to spend as much as you can and you will end up eating at some fine dining places if you have the chance!
It's hard to eat on the road. If you don't get the free breakfast, getting two eggs at the hotel often costs up to $15 ($25 if you are staying in a nice hotel)! Also, you aren't allowed to take liquids through airport security, so you are often subject to the inflated $5 bottle of water at the airport. $85 may seem like a lot at first, but when you are traveling late nights and forced to buy at the airport and use room service at the hotel, it goes FAST! It's hard to stay within perdiem.
There are a lot of CRAs out there that don't take care of themselves and end up being fast-food bandits. I just can't do that to my body, and of course, we gotta fit in those airline seats!