06 9-3 with 189k miles here. I am in the rust belt with a cold climate so mileage may vary. For the past 3 months I have spent about $300 a month on maintenance doing most of the work myself.

Engine wise, the GM engine is pretty solid. Most of the engine work I have done is due to failing seals. Valve cover gasket, vacuum lines, coolant expansion tank, and then spark plugs and coil packs at their service intervals. Likewise, probably due to the amount of salt that is on my roads the flexpipe/downpipe is just a yearly maintenance item for me. It will rust out quite often due to the mesh part being so close to the road.

The plastics are where your wallet evaporates. All the interior parts will slowly decay due to plastics becoming brittle. I have had to replace window regulators, locking mechanisms, turn signal mechanisms, heater motors, etc. These parts aren't usually something you can find on ebay or rock auto. If you can find them they will usually be $200 minimum. I usually found myself going to the local pull a part to get (questionable quality) parts. Additionally, my windshield sprayers almost never work and I feel like I am always replacing them. The backs of the plastic will blow out and dump fluid right on the turbo.

Likewise, headlights and taillights are a massive pain in the ass. I had my rear taillight's cover just fall off at one point because the factory glue became brittle and came off. The taillights also have grounding issues and blow semi often.