With the widespread proliferation of smart phones and smart pads, it seems that not too far from now, 3G will follow the route of regular cell phones. There will be this day when 3G (or similar future technology) becomes a necessity in our society, just like how dependent we have become of our cellular phones today. Everyone will have a 3G plan of some sort, for the myriad devices that each of us owns. While 3G cell phones may not make sense for everyone, but when 3G capability is put into the very devices that each of us really care about (be it an audio player, a netbook, a portable video player, etc), then it'll make sense for everyone to own a 3G data plan.
When 3G finally becomes an integral part of our lives, the cost of such a plan would also diminish to an extent where it is affordable for everyone due to infrastructure improvements and cost reduction technologies. It will cater for everyone's needs. Of course, those with greater 3G needs will have to fork out more for that piece of luxury. But it should not cost more than 10-20 bucks a month for a basic plan that is sufficient for most needs.
Then again, what is sufficient? The greed and hunger of men can never be satisfied. When one finally settled down with a basic plan that requires no more than a few GB a month, one will start thinking what more one can do if one has a few more GB to use.