Well, my DD11 does have one of those, but we don't allow her on the internet on her phone, but yes, it would be $10/month. (and for the record, the $120 a year for internet for an 11 year old to simply play on is why, not because we don't trust her. She uses the internet on our computer, the laptop and her DSi. She doesn't NEED it on her phone too).
DH and I both pay $10/month for our phone internet with T-Mobile. Of course we don't *need* it either

but it sure it nice, and we are the ones earning the money. When DD gets older if she wants to earn money and buy phone internet, then she can. (plus we dont have DSi's - LOL!)
I don't know anything really about pre-paid plans. Sorry. We have the "Even More for Families" plan, and pay a flat rate each month (unless we go over, which we never do. We use maybe 1/4 of our minutes each month, so it's not like we have to ration them).