Data is Data. It will use your data, exactly how much is the question. Very hard to say. I use BBM, FB, Twitter, Browse a lot, GPS, and a few other things and I seldom go over 200MB. Very seldom actually.
Personally, I don't think you will ever exceed you 500MB.
Thanks for the speedy reply people
I'm getting it first thing monday as its my first BB I thought I'd get an entry level one and yake it from there.
one more thing am I to take it that BBM is free internationally as it comes out of my data usage like an email ( got a few friends in Blair OK )