Your only option is going to be what the operators wanted all along; pay their normal price.
The cheapest solution would probably be a family plan additional line with a data add-on, but you are looking at about $35/month for that on T-Mobile, without 3G (since there are currently no T-Mobile 3G cards). AT&T has a PDA only add-on for $30 (when added to a family plan, $35 stand-alone) which may be able to be used on a data card, but the "official plan" would be $60.
You got a good deal for some time, but the carriers got smart (and greedy) so you are pretty much out of luck.