Chile has some of the highest ATMs fees compared to most South American countries I've visited. The best option would be to avoid using ATMs altogether but that's very difficult.
The cheapest way to exchange currency ( Euros and dollars have the best exchange rates in Chile )is to change cash at forex bureaus ( casa De Cambios) in Santiago. Most casa de Cambios in downtown Santiago are along Agustina street
Banks and ATM fees
ATM fees in Chile depend on the location of the ATM rather than the bank itself. ATMs in high-end malls, shopping centers, and hotels charge more than ATMs of the same bank in downtown Santiago.
Banco security is the only major bank in Chile that I know has free ATM withdrawals across all its ATMs. If you have a US-issued card it's safer to stick to Banco Security.
Most ATMs have a withdrawal limit of 200000 Chilean pesos. Banco Estado is the only bank that will allow you to withdraw more than 200000. You can withdraw an extra 600000 pesos at their ATMs but at an extra cost of 4000 pesos.
I also noticed that most ATMs only accept 4-digits. Stick with Visa, Mastercard, and Payoneer ATM cards, these are the most common in Chile.