a couple hundred dollars tops

~50$ each, so 250$ for a set with your spare (you should put one on your spare).

They are expensive to get installed on their own, but if you're getting new tires/a mount and balance anyways; then it is free. Maybe a small charge for programming if you don't want to do it yourself.

Edit: spelling is hard

I tend to notice when tire preasure drops as it tends to pull the car to one side, cause a lean, and increase the road noise. The sensors often need to be replaced several times throughout the life of the vehicle so the cost is certainly more than a few hundred dollars in total. In my area cars must pass a safety when transfered between owners, these senors must be working to pass the safety if the vehicle is equiped with it regardless if you want the feature or not.

A few hundred bucks to pay someone to replace a sensor you may not need is certainly a significant sum of money to some people, especially those buying used cars as their finances are often already fairly tight.

When I bought my most recent car I tried to get as little electronics as possible to reduce maintaince cost on bells and whistles I don't need. I have manual windows, a manual transmission and very little bonus features like blindspot warnings and tire pressure sensors. My maintaince costs are less because I dont have to constantly replace sensors that detoriate from road salt, wide temperature ranges, and general wear.