How much money do you think you're saving by bringing in your own cables and housing? Usually LBS's have rolls of bulk housing that they'll cut to length, that stuff is a lot less expensive than the prepackaged shimano / jagwire / campag / nokon etc cables. And it works just as well, unless you have a specific application that needs nokons for some reason.
I always buy cables & housing from the LBS and do the work myself, cuz I like doing it and I'm cheap.
Sometimes quite a bit. I'm new to my area and new to the LBS it will be the first time I'm spending so much as a dime for a gel pack. In the past at other LBS's and the quote I was just given have my tune up over $200. Not that my bike isn't worth considerably more but the sport is expensive enough. I've been to stores in the past that somehow only have the most elite cables, housing, and chains in stock and I end up having to take out a small loan (slight exaggeration) for a simple tune up. I do a lot of work myself but often like to have the LBS do the tricky and annoying stuff. I suppose if it is bad form and stresses a relationship with the local guy that can do complicated stuff at a much better quality than I can it's worth the extra few dollars every year.
I still lapped everyone on the couch!