2 hours a day, 5 days a week, 4 weeks a month is 40 hours of browsing minimum.
250 MB / 40 = 6.25 MB an hour. According to Web Site Optimization: Speed Up Your Site website optimization web speed optimize web site performance company, ipadforums.net frontpage is around 625KB, while the first page of this thread is 323KB.
So for example, if you followed the structure of this website, and went to the main page, then iPad Forums, then General Discussion, then clicked on this thread, you would use 1.7MB of data.
So yea, the 250MB plan is pretty worthless.