you should be able to get a good cog and lockring for well under $40. I think mine (Surly cog with Dura Ace lockring) came out to about $35 installed at the LBS (didn't have a lockring spanner at the time so I figured I'd let them do the dirty work).

Try this:

There are some OK prices for Dura Ace cogs and lockrings on Amazon, Niagara cycle works, JensonUSA.com, etc. You should look to spend about 30-35 bucks for a really good cog+lockring setup (plus shipping). Also check the Ebays...



***I guess I don't know if you're running 3/32" or 1/8", but these should be a good start. I also got a decent chainwhip and lockring spanner, but some people live and die by the Rotafix method. Lockrings are, however, your friend IMO...