I just finished replacing the battery pack heater on my 2018 Model S. I’m at ~130k miles, and it went the other day preconditioning before Supercharging. Tesla charges $800-$1000 in labor to do it, but I’m comfortable performing the high voltage isolation and swapping it myself for $200 in parts. Moral or the story is even if you get them cheap, be prepared to spend a bit if you can’t do the work yourself. The 3 and Y are more robust around thermal system reliability (heat pump conditions both battery pack and cabin, for example) but the complexity has a cost regardless.