Also: some aftermarket wheels are not drilled for this, so you're essentially paying them to have it mounted properly which may involve drilling the wheel barrel to add it on.
This is mostly a problem on multi-piece wheels.
See the pics for info:
https://www.google.com/search?q=tpms...w=1280&bih=939
You don't need this though, if you monitor your tire pressure manually and/or on a regular basis.
-alex