A little older now, but:
2GB for $25/mo with AT&T, $10/GB overage.
$30/mo for 2GB, with $15/GB overage.
Data prices do seem to have gotten better:
where they reduced the plan from $80 to $65 for voice+2GB data.
Indeed, last year to go from 2GB to 5GB cost $20, and now costs $25.
From the CBS article, "With that plan and voice service, a smart phone could cost as little as $55 per month before taxes and add-on fees, down from $70 per month."
According to AT&T's planner, that means in SIX YEARS, costs haven't changed at all, overall.