This tip below may help....I am not the one giving the advice written in blue lettering below but I like it very much because it sounds feasible especially since you are not in a contract with a service provider. However, you would have to go to a free Wifi place such as starbucks (there are many more) and turn some of the items you turned off back on again.
It would be helpful if you would try it and post the results and let us all know if it worked for you. Thanks.
I've been vacationing in Florida 4 weeks now, I come out of Germany. Here's what I had turned off in the phone's Settings and have no data charges since being in Florida:
Notifications - Off
General - Network - Enable 3G - Off General - Network - Cellular Data - Off
Mail, Contacts, Calendar - Fetch New Data - Off Mail, Contacts, Calendar - New Invitation Alerts - Off
Messages - Repeat Alert - Off
iPod - Lyrics & Podcast Info - Off
Mainly you want the network 3g and data turned off. The place I am staying has free wifi so when within range I turn a few of the above settings on to catch up on things.