Mine are almost 9 years old now and still working great. I don't replace anything just for the heck of it, unless I have something torn down already. I would't call $10-$13 for each GP expensive either!! Of course while your in there I would change them all. I would only use the Ford units, don't use the Bosch GP's they are junk. If you look in the maintenance section I have a sticky on how to troubleshoot the GP system.
NCH