International Data Plan

I am planning to go to the Philippines for one month and would like to bring my bb. I wanted to use it mainly just for email and bbm. I am planning to get the $25 data plan and then add the $50 intl data plan for 10MB (for a month). Will this be enough (only text email and bbm)? How is data being charged for intl use? Is it based on the data being received and send and not based on the connection or roaming?
I just don't want to be surprised with the bill I will be getting....
Thank you for your assistance