You're not paying twice as much for the same thing.. you're getting a better network, with unlimited roaming, and in many cases, EVDO revision A coverage...
However, you're right. It's not worth 3x the cost. I went into AT&T the other day to find out about switching, and my bill would have been $162.xx per month including taxes and unlimited everything... If I buy the phone for retail, I don't have to sign a contract, but that's little consolation.
I'll switch when I can get unlimited for $100+tax... an extra $50 for data and text is just too steep of a price.