I don't personally "make" money from my per diem. That's theft/fraud. I have caught many people funnelling perdiem and corp card money into their own hands, and have had a few sent to prison. Things like buying gift cards, then selling them to a broker etc. Sometimes even silly things like shopping and then misrepresenting the expense reason.
But, I did work for a company that let me roll my budget each month into the next month, so if I was frugual at the start of the year, I was taking some very luxurious business trips by 4th quarter (And some very budget ones in 1st). By the last quarter I was going full fare F and staying in resorts. And it was all within policy and the allocated budget, and never entered my hands personally. As you suggest, much of the frugality was centered around buying meals in supermarkets and using Hotwire for cheap discount rooms.
Most companies I have worked for have allowed me to retain all airmiles, points etc into my personal accounts. This includes points from the corp Amex, EQM's, my code share alliances etc. When I was on vacation or left the company I took them with me so I didn't have to start over at the next company. That's how I buily top tier status on some carriers, cars and hotels over the years. When I went back and forth to Asia several times a year I racked them up very quickly. But again, that's per company policy and doesn't cut into profitability. It's a perk.