I just bought a set of four tires and had them changed for the first time, and went through this. There are two different things being discussed here. There are (1) the sensors (which include the valve stems; do a Google image search for "tire pressure monitoring system"), which do NOT need to be replaced, and there are (2) the washers around the valve stem, the nut that holds the valve stem to the wheel, and the "innards" of the valve stem itself (not the sensor), which can be replaced at tire change time. Replacing (1) is about $180 per tire. Replacing (2) is about $18 per tire. Go to a tire store that has done this before and know the difference.
Thx dypyang. Googling per your advice, and Wiki'ing, it appears there are batteries inside the sensors that will require sensor replacement after 7-10 years. I guess that's the time you replace the stems with standard valve stems.