The car uses a transversely mounted V6 engine. The front plugs are easy to get to, but the rear plugs are not. It varies from model to model, but the engine may have to be detached from its mounts and rotated or lowered to get at the rear plugs. A lot of disassembly can add to the cost. Call your dealer and ask what it costs and what exactly they do. As Joseph mentioned, the plug wires should be replaced and maybe that is part of the cost. You also should get another estimate or two at independent garages to see if the cost is less for exactly the same work.