Where the **** is all this math coming in to play? The OP has at&t and will continue to have service with them. It's no different then his normal voice plan. It's not extra unless he doesn't have data in which I'm sure he was planning on doing so anyways. He is eligible to upgrade and two years does nothing to him if he is happy and is staying with the carrier. FAIL!
To OP: Yes, it's cheaper. Go for it. Enjoy.
in theory, he could buy a 3GS, then switch back to his old phone, call ATT and say he is no longer using an iPhone and ask them to remove the data plan. They will then remove the data plan, and he continues his service as it was before he had the iPhone. Officially though, the 3GS cannot be used unless it is activated with an ATT SIM, making what he wants to do technically impossible
there is a work around.
Listen closely, OP
-First, buy the new 3GS, sign your contract, and activate it. Then, wait a few days.
-Then, after the proper tools are released (should be soon), you will have to jailbreak your 3GS and unlock it with UltraSn0w. Once you have downloaded and installed UltraSn0w like said here
http://blog.iphone-dev.org/
then reboot the phone and remove the SIM. The iPhone should operate as an iPod touch, just without cell service
-Next, remove your SIM from the iPhone and switch back to your old phone. Call ATT and tell them that you no longer are using your iPhone, and would like the data removed
-Be careful not to update your software or anything until it is said to be safe on that site listed above (check in there every day after a new release comes out to see when they say it is ok and how they say to do it), as this will ruin your jailbreak/unlock
however, you will likely still have to pay the data plan for the first month. Theres also the $18 upgrade fee, and the $18 activation fee (you amy be able to have this waived). So your not saving $100. At best you save $82, possibly only $52, and at worst, only $32. But still, you get a camera, voice controls, and a more powerful device for all that, and just a little extra set up hastle
hopefully this all hasnt been too confusing for you, or anyone who wants to do as you do