2001 Jeep Cherokee Oil pressure gauge

My guess is you have a faulty oil sending unit.

You can either check the actual pressure with a gauge or replace the sender.

The high idle may be the first thing to go after in the event that it is causing a higher pressure.

Start with checking the throttle body to make sure the cable is allowing it to close all the way, or that the throttle plate is not carboned up and not allowing the closing of the plate.

But to answer your question, I would expect a 700-850 cost for the pump.