dschl

Here's a rough guide from a site I was browsing today:
http://www.oakbaybikes.com/techstuff.htm

Use your judgment, based on your riding conditions, distance, abuse/care of the bike, and regular inspection. Last summer, I rode an old mtb commuter home in heavy rain, with plenty of grit on the road shoulders, and found out that the bottom bracket seals are probably not doing their job. I had just repacked it with fresh grease, and by the next day, it sounded like I had mixed the grease with sand. That bike needs moving parts relubed more frequently than others in my fleet.

The $125 for an overhaul seems like a lot of money to me, but then I've been (mis)maintaining my bikes for several years now. The cost is probably commensurate with the amount of time it takes a mechanic to tear everything down. Of course, for $150 at MEC, you can buy most of the tools you need. Another $125 gets you a repair stand. The next $125 gets you the oddball tools, and all of your consumables for the next few years (chain cleaner, citrus-based degreaser, ball bearings, chain lube, grease, etc). And by having the tools and skills to do it yourself, you're more likely to be able to deal with problems that arise during journeys, and avoid leaving your bike in the shop for 2-3 days every time something goes kaput.

As for the headset, a good LBS should have a bunch of old parts on the shelf to cannibalize, unlimited new parts available by special order, and they should be able to come up with a solution for just about any problem your bike might experience. They might not be able to do it cheaply (ie, perhaps you'll require a new headset for $20-50 as different brands might not have interchangable seals), but they should be able to offer you a solution, not a shrug of indifference. There is a price to pay for expertise, and when I'm scrounging for parts at my LBS, I just pay what they ask.