A MKII (non LR) I believe runs about $75-$90. It was $60 a few years ago.
To replace,remove the wind sensor, pull the temp/rh out of the tubing carefully with needle nose pliers.. The sensor itself is about 3" x 1". Looks like a small circuit board with the standard Rainwise phone jack plug on one end. Takes about 10 minutes to replace depending on where/how your station is mounted.
Call Ralph for he should be able to troubleshoot for you. Some believe the problem can also be related to the cable that runs from the main motherboard to the temp/rh sensor.
Good luck