100 pesos is approximately US$2.50
Anyway, I'm from the Phils too and my sister has a Galaxy Fit and this used to happen to her (she had the phone for a couple of weeks before I was able to get at it). The mobile data was open so I disabled it and it does not eat the prepaid credits anymore.
If you use your phone to surf the internet, you will have data enabled. However some apps need to sync in the background. Each time these apps connect the internet, it uses up credits (load in the Pinoy slang). What you can do it go pull down the notification bar and slide the toggles there from right to left. You will see the toggle for "mobile data". Turn it off. You can just turn it on again when you want to use mobile internet.