It's difficult for any shop to quote an exact price for a valve job because there are so many variables involved. The ultimate price you pay will depend on the engine application, the parts replaced, and any additional repairs or work that may be necessary.
The least expensive valve job would be one on a domestic V8, V6, straight six or four cylinder engine that required no crack repairs or head straightening, no new valves, springs or other hardware, and no special tools. The cost of the valve job might be as low as $100 for the machine shop portion of the work, plus $60 to $75 for gaskets and antifreeze, plus $250 to $400 labor to remove and install the head.
On the other hand, if you have an import car with dual overhead cams, four-valves-per-cylinder and a lot of fancy valvetrain hardware, you might be looking at a total bill of several thousand dollars. Parts typically cost much more for import engines as well as performance engines and diesels. The labor to do the valve work is also much more involved on these overhead cam multi-valve heads. (Of course, they didn't tell you that when you bought the car. All they stressed was the fantastic performance of overhead cams and multiple valves.)
You should know that most garages and dealers do not do their own machine work on heads. They send all their heads out to a machine shop that has the necessary valve and seat refinishing equipment.
If you're an advanced do-it-yourselfer, you can pull your own cylinder head and take it directly to the same machine shop and save the labor costs for removing and installing the head. But pulling a head on a late model engine is no task for a novice. There's a lot of stuff that has to be removed and reinstalled correctly. What's more, on some overhead cam engines installing the timing belt can be tricky. So if you have any doubts, don't tackle such a job without professional help.