Are you talking about just the telephone or the telephone with Nextel service (from the US):
-If you are just bringing the phone to get service in Peru, you will need to unlock it. However, you can usually get it unlocked in the phone repair shops in Peru (not Nextel stores) for less than US$10.
Once you go to the Nextel store, you can just get the chip for the phone and either start a contract or buy a rechargeable chip (you have to buy prepaid nextel cards to add money to your phone's account). By the way, you can also buy unlocked nextel phones on ebay or amazon.
-If you are bringing a phone already with service Nextel/Sprint. Making regular calls will cost you around $2.49 per minute due to the international roaming.
If you add the "International Direct Connect" add-on which is $10 you will be able to use unlimited radio in the US, Mexico, Peru, Argentina and Brazil though.
Also, if you get free text messaging in your service, you can use that to send messages to people in the US without extra charge.