How much does internet service generally cost for a cell phone?

[Ð4MÃN! (End the Fed!)]

I'm looking to purchase internet and texting service on a pre-existing cell phone plan with T-Mobile. I've never had either of these services before, and was wondering how much they would cost.

I've read that texting for a family plan(which is what I'm a part of) costs $20 bucks per month, which isn't bad.

I plan on getting a Samsung Highlight or T-Mobile Tap, and was wondering how much does internet service generally cost?

And what is 3G?

Thank you!
I live in the United States...in the Northeast.