For the last several years Peruvian currency has stood at roughly three solés to one American dollar, making the cost of travel in this country especially economical for budget-minded tourists.
With this three-to-one ratio in mind, you can calculate costs accordingly, and frequently housing, food, and transportation costs are cheap; you can buy a decent lunch for $3 in cities such as Lima or Cusco.
Indeed, if your intention is to see as much of Peru on as low a budget possible, your daily costs can go for no more than $15 a day.  But that means hostels with shared bathrooms and taking some of the cheaper (and riskier) busses between cities.  If you are willing to spend at least twice that you can upgrade your comfort level and avail yourself to such activities as museums and guided tours.
Travelers with few budget restrictions will be able to glide through Peru, in that there are many top-quality hotels, restaurants, and tour operators that are relatively inexpensive by first-world standards.  Some actually charge first-world prices as well for millionaire-level comfort, but sometimes such a price is little more than a reassurance who will always assume that a higher cost is a guarantee of quality.
The following banks have an agreement to exchange American Express travelers checks with no commission:
-- Banco del Crédito del Perú (BCP)
-- Interbank
-- Scotiabank Perú
Some branches of Interbank will also exchange other brands of travelers checks (Thomas Cook, MasterCard, Visa, Citibank, Citicorp). This differs from city to city, so check the local branch. In some cities Banco Azteca is open 365 days per year and into the evening. It exchanges U.S. dollars. (Though the rate seems to differ from city to city, for example in Piura the rate was bad but Nasca and Tacna had a much better exchange rate). A number of businesses handle Western Union transactions, the most convenient is Scotiabank and MoneyGram services are available at Banco de la Nación. Both banks fulfill money wires in soles or U.S. dollars.