Sadly, the extra 200$ is a fair price. They recoup that money over the course of your contract... Perhaps they could prorate the discount for the amount of time you're through your term?

Further, the only reason they were able to offer the discounted price when shifting from 2g to 3g was the additional 10$ a month they made off the new data plan. For the 3gs, they'll actually lose money on upgrading customers because their bandwidth costs go up.