Here in the UK, my mobile phone bill is £3.33/month and I get 500MB of data with that.
EDIT: I do this by using T-Mobile PAYG and buying their 6 month Internet booster for £20, twice a year. I've managed to convince all my regular contacts to use Kik Messenger instead of SMS, so I don't have to pay for SMS. I very rarely make phone calls, but I have VOIP with localphone.com when I have to.