You must mean the Thermostat. If the thermostat is bad it can cause over heating. Not sure about the 2001 pathfinders. They went to a timing chain and different setup for the waterpump and thermostat then my 98 pathfinder I use to own. For the 98 pathfinder the thermostat was very easy to replace. No more then 1 hour labor plus parts. $300 may be reasonable it’s a couple of hours labor.
As for trading it in…You’d be far better off selling it on your own. You should be able to get a lot more then $2200 for that vehicle.