The airbag and steering wheel angle sensor was the same as that found on the 2 spoke wheel but the 3 spoke frame was wider all round and had thicker hand grips making it feel nicer in use.
These retail for about £600 to £700 new from smart but they sometimes turn up on eBay and sell for between £200 and £250.
Being leather you need to check what you are buying if you do get one second hand.
The thing you can't check is the ESP cartridge on the back (angle rotation sensor).
These aren't hard to damage beyond repair as the calibration is very sensitive.
It only takes a slight movement of the cartridge when the wheel is removed to cause issues.
Some can be repaired, some die for ever. You can't buy them separately but you can swap them from your existing wheel.