I have traveled to many countries around the world. Some accept credit cards. Some only deal in cash. Some accept US Dollars. Some only accept local currency. Nowhere, however, I have dealt with a system as weird as Argentina. There are myriad exchange rates.

Consider this data based on my real experience.

1. Official rate: 1 USD = 226 pesos. This is what one gets in official exchange places. And probably while using the ATM. It is a bad rate though. Do not use the ATM.
2. Blue Dollar rate/Black-market rate: 1 USD = 466 pesos. I converted a 100 USD bill at the rate of 1 USD = 460 pesos. The rate offered for a 20 USD bill was 1 USD = 453 pesos. 10% of USD bills in circulation in the world are in Argentina!
3. MEP rate: 1 USD = 415 pesos. This is the rate I got when using my Visa credit card. For my Master card, I was charged 1 USD = 226 pesos. And later, got a refund, making the effective exchange rate to be 1 USD = 395 pesos. So, the MEP rate is ~10-15% lower than the blue dollar rate.
4. Hotels/hostels were offering an exchange rate of about 1 USD = 440 or 450 pesos.
5. Western Union gives a blue dollar rate except it charges a USD 5 transfer fee. The first transfer, however, is free. I didn’t use Western Union.
6. Many businesses, especially, taxis will charge a 10% credit card payment surcharge.

The Argentinian inflation rate is at 100% a year! So, no foreigner wants to end up holding too many pesos.

So what to do

All of these various issues cause significant consternation. Here’s my recommendation for what to do.

1. If you are staying in a big city like Buenos Aires for a few days then just use credit cards. You will lose 10-15% as a price of convenience.
2. If you are going to be traveling inside Argentina, for example, Patagonia or Ushuaia then bring 100$ bills. And convert them to pesos in Buenos Aires at the blue dollar rate. How much money should you carry? I would say about 30$ per person per day would suffice. This excludes any airline tickets which are hard to book with cash anyway. So, for a 10-day trip in Argentina, I would recommend converting three 100$ bills.