I just replaced both my mirrors on my 2002.  The cheaper one I could
find was at amazon.com about $15 for the left and $30 for the right.

You need to have a T20 torx screw driver as well as 3/8 socket wrench.

http://www.myfordfocus.com/howto.htm

http://www.myfordfocus.com/how-to/mirrors.htm

give you instructions but I needed to be shown how to Detach the trim
panel with my bare hands in step 1.  I lower the window, grab the trim
panel with both hands and pulled towards the inside of the car and to
the rear.

Step 5 required the special torx screw driver to unscrew from the
door.  I also remove one screw from the inside door panel so I could
pull it away far enough to get the wiring through the door.