$65 is on the high end, but average for retail price. obviously buying them yourself online you can get them for as low as $20
i'm not completely sure about this but it would seem to make sense--- if the sensor was not working then you would not have a light on because the sensor only tells the car that the pressure is low, not if its working or not, so non working sensor = pressure is fine, as far as the car is concerned.
as i recall, i did not have a light on even when i had one sensor removed (so only 3 on the car).. also because i was told it couldn't be used (i think they just didn't want to rebuild it)
yes, it is illegal to not replace it.. federal law mandates tpms sensors on new vehicles
"but if it was just leaking from the valve in the stem, that could also be replaced without necessarily replacing the unit (i think)"
the metal valve stem on the oem sensors cannot be replaced themselves.. it's the whole unit or nothing. when the time comes to replace your sensors you should go to a shop that will sell you the new style with the rubber valve stems that can be replaced. that way you never have to buy another sensor (at least until the battery is dead).
the core of the valve stem can be replaced, if that's what you meant.. and that is part what is replaced when they service the sensors, but the whole stem itself cannot be replaced
btw, the oem sensors with the metal valve stems will corrode and become unusable before the battery even dies.. at least one of my stems is starting to get a bubble and could burst open .. but they are only 3 years old of the 7+ year battery life