vandykes4: according to my understanding of the various comments made, this is what you will have to pay:
USD25 for your flight to Quito, unless you stay within the airport and within the transit area, in which case nothing;
10 soles (about USD 3) for your flight to Cuzco (same thing at Cuzco for return flight);
USD25 for flight back to Miami (since you are flying from Cusco rather than an international origin point, the fact that you are in transit will not help you)

You should not ahve anything to pay in Quito since you come back the same day.