When money is discussed in a book, what currency should be used for a book translation covering many countries? For book translations, what currency does one reference when translating for one language that covers a bunch of countries? 

What I am alluding to is this: suppose someone is referring to something as costing four rupees. In the English version of the book, there is an editor's note explaining that it is the equivalent of U.S. .06, in order to give U.S. reader's an idea of how little that amount is. If you are translating the book into a different language that covers a lot of geographical territory, say, Portuguese, what currency would you use for the editor's note for areas and countries that are not pegged to the dollar, and use a different currency in every country where Portuguese is spoken? (i.e. Brazilian Reais, Euros (Portugal), Mozambique Metical, Angolan Kwanza, West African CFA franc (Guinea-Bissau), U.S. Dollar (East Timor), Central African CFA franc (Equatorial Guinea), Cape Verde Excudo, São Tomé and Príncipe Dobra.) The book could be available and read in any of them, so there isn't just one country to consider; however, different versions will not be available for each country (there won't be a "Portugal version," am "Equatorial Guinea" version, etc.)--there will simply be a Portuguese version for anyone who speaks Portuguese. 

This doesn't just apply to Portuguese; this will be problematic in many other languages too.

It's a nonfiction book, if that matters.
I've had similar issues with translating currency in the past.

I believe that as a general rule of thumb, if the text has a worldwide audience, using US Dollars is the best choice. Many countries will have a general grasp on the value of USD and those who don't can simply use conversion tools to guide them. If the book applies to a more narrow geographical area, you can use the currency for that area (e.g. Euros for a European Audience)

No currency is accepted and understood worldwide. However, US Dollar is probably more global than the rest.