I have a Movistar PAYG chip in an unlocked phone (Nokia N95) I bought with me and use it for email checking when I'm not in front of the computer and occasional web browsing in the line at the supermarket, google maps etc. With two of us using it, we usually put on 30 pesos a month, which gets bumped up to 45 as you get 15 free, and if it's doble dia it's bumped up to 60. I very, very rarely need to put more than this, but we don't make an awful lot of calls on it either. After the month is up, the credit left over is locked until you top up again, but as long as you don't leave it too long you won't lose that credit. If you put less than 30, it lasts less than a month before you have to top up again.