That's a common trick. On a 2 GB plan, if I use less than 2 GB then I pay NZ$60. If I exceed 2 GB then it's another $30 (ie $90 total) for an additional 2 GB. On the other hand, if I sign up for the 4 GB plan in the first place then it'd only cost me $80.
My operator also offers a 500 MB plan that might be better suited to some users, and it's a shame that AT&T doesn't offer it. It's marketed as "500 MB for $30" but it's not charged in bulk. You pay 10c/MB until you hit 300 MB (ie $30), and then the remaining 200 MB are free. That's a great system for me since I can use 20 MB one month and 200 the next month.
1 NZD ~ 0.68 USD