I traveled true Peru and Mexico a lot and sometimes you have to catch a cap. Depending from where you are and where you are capdrivers will recognise you as a foreigner and depending where you are try to charge you more for the ride as they would do with locals.
I use the following stradegy to get a normal price. I always ask a random person who is not a cap driver and not a tourist what they would think how much it wil cost them to make the trip. This will give you an idea what the trip would cost for a local person.
Then dont ask the cap driver what it will cost but tell him what you want to pay.
This stradegy works really good in Mexico and Peru. i dont know if this works as well in other coutries.