It's not worth hunting for the cheapest rates since they all are obligated to charge the same amount, which isn't much. I paid 180 pesos per document, no matter how long it is.
I could recommend my translator in Balvanera, but if you're in another neighborhood it's best to check the list of certified public translators online and find one in your area. You have to drop off and pick up the documents and pay for the translation. Then it's a matter of having the translation certified on Corrientes near Callao.