In most cases, anyone from South America will be considered a "national" even if they're from outside Argentina. I'll relate a few examples of what I experienced and I have to stress this didn't upset me and I paid what was asked of me.What about when a poor farmer from Paraguay who visits and has to pay more than a local?
At the entrance to the Perito Moreno Glacier park, the mini-bus stops and a girl comes on with a clipboard and asks for our passport or ID. A big sign on the side of the road lists the prices and yes, the price for me is 30 pesos. Next to me were two guys from Chile. They paid 10 pesos each, which was obvious because the girl didn't have enough change and asked me for some. Same with other people from other SA countries. In front of me was a couple from Germany and they were a bit annoyed and asked why there's such a price difference and why anyone getting a discount didn't have to provide a DNI (we were writing our info on the clipboard paper, so it was also obvious). She said that they were "nationals" and made a quick exit from the Bus. One of the guys from Chile apologized to the German couple and said this is "how it is" here.
In Puerto Madryn, the same thing was going on with the big sign showing the different entrance fees. Again, it was only the out of SA tourists paying full price. On the clipboard sheet, there were ID numbers listed but I never saw anyone actually show an ID other than those of us with passports.
Now the interesting stuff. While in Bia Blanca, I was looking for a "free fork" (all you can eat) restaurant and found one listed in my guidebook. It's a nice place right off the main plaza and there's a girl outside holding a menu asking people to come inside. I stopped and talked to her and found out the price is 25 pesos. It's actually on a sign right in the window. While I was looking the sign though, I noticed in small print and in spanish that if you provide a DNI number, you get a discount of 50% per person. I asked her if this is only for Argentines and she got a bit nervous. Well, she said, Argentines and nationals. I did go inside by the way.
The same story in Bariloche, again at a free fork. If it's not in the fine print, most nationals will ask at the door. At least that's what my girlfriend said after I got back and we laughed about all of this. It was great taking her to a restaurant by the way because she'd grab the bill at the end and go over every detail. I would've been double-charged a few times if she wasn't there.
I also saw the same sort of thing at stores in Rosario, where they'd have sale prices for people that can present a DNI. Just like we might have sales in Canada and you get a discount if you've got a member card at a grocery store, so it's not like I was getting upset by all of this. Airline tickets... I don't want to talk about.