Would someone explain a usage scenario for the $15 250MB plan? It seems to me that you couldn't really do anything on that.
I use no where near that amount a month on my iPhone and I wouldn't expect to use dramatically more on a 3G iPad.
The trick is having wifi access in places you spend lots of time. I have wifi at home, use wifi at the companies I work at, use wifi in coffee shops and use wifi when stopped in motorway services. So I don't actually download much on 3G at all.
I'm sure there are people who do use more, but for example when Ars Technica polled iPhone users most people said their data usage was under 250MB/month.