TL;DR - Use ING (Visa) on purchases for about a 40% discount on everything in Argentinian Pesos, including ATM withdrawals.
Nothing really that complicated here.
Unfortunately, Argentina's Peso is inflating at at about 100% per year.
This makes things difficult for the locals but opportunistically cheaper for visitors.
As of writing, the official exchange rate has the Australian Dollar buying about 132 Argentinian Pesos.
And the USD buying a smidge over 200 Pesos:
But the official exchange rate does not describe the exchange rate on the ground. Specifically, on random street corners with men yelling "Cambio! Cambio!" or even at coffe shops that will happily change your money.
The unofficial exchange rate, the 'Blue Dollar' is more generous. It is explained well here. The rough explanation being that in times of rubbish governance, the population will determine the value of a stronger currency.
So with a dodgy little Blue Currency calculator (the rate changing every day, of course), we can see that changing your money in the street or at a coffee shop can give you:
So your USD may get you 200 Pesos on the official market compared to 369 Pesos on the Blue market.
Now lets turn back to AUD.
Withdrawing 10,000 Pesos from an ATM should cost me $75 AUD:
But if I check my statement from my little ING Card that I use for travel cash, I see:
So I bought 10,000 Pesos at a discount. Essentially 60-65 cents on the dollar, depending on the day.
And the same has remained true for accomodation payments listed in Pesos that I have made with the Visa Debit card.
It seems that the Visa network is being allowed to access the Blue Dollar rate. And the ING card I use also refunds international transaction fees and ATM fees (unfortunately only for the first 5 fee-incurring transactions, I believe) as well, which has been a long-term offer not specific to Argentina. I honestly don't know why it is the case that the Visa system is allowed this privilege.
The Amex system is not linked into this, unfortunately. As we learned after checking the statements on a couple of early Amex purchases when we arrived in Argentina. So I have stowed away my Amex card that I would have otherwise been using for travel purchases (points, security etc) for the duration of our time here.
That's it, basically.
So, once again, use ING (Visa) on purchases for about a 40% discount on everything in Argentinian Pesos, including ATM withdrawals.
Addit - As a travel spending nerd, it has been a bit annoying in that I have had to manually add the Blue Currency rate to the spending log in the spending App that would otherwise do the official exchange rate calculation automatically.