I find that my repair decisions are based on convenience rather than price...if you have to pull this part off and that part off to get to the offending component, (assuming a older, high mileage car, and the said parts are original, or aging), you might as well replace those as well.
For all of the labor involved, it's easier than just replacing what's needed, only to have to go back a few months later to replace other parts (and expend hours of labor) that were readily accessible earlier.
So when I replaced my alternator, I went ahead and sprung for a new serpentine belt. If I ever have to do the head gasket, I would probably do the seals and timing chain as well? Get the idea?
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car