I got a set of chrome OE wheels about a year ago. To break down the tires off the old wheels, mount the sensors and tires to my new wheels, and balance all the wheels cost me 100 bucks at my local Discount Tire.
To get your sensors out of the old wheels they need to break down one bead and I think they can reach in and pull out the old sensors. That shouldn't cost much at all, unless you want them to mount some new valve stems in there with no sensors, and then rebalance the wheels - it still shouldn't cost more than 100 bucks!
If you're going to keep the old set of wheels/tires for HPDE's or auto-xing or something I think you should just leave the sensors in them. Even if you're going to sell them they might be easier to sell all mounted and balanced with the sensors in them.
If you want new sensors, they will cost around $200 or $220 from forum vendors. Check with Bob (
http://www.houseofwheelsonline.com/ ) or Gene Culley (
http://gmpartshouse.com ).
Our sensors are made by Schrader and Bob has them direct from Schrader which avoids the GM tax. Gene's are GM parts and cost a little more.
Or, I've got a set of used ones for $160 shipped. Problem is I won't be home until after Jan 1st.
Bob