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The combi valve is a small valve mounted on the engine that opens up, allowing air to blow into the exhaust on cold startup to aid in catalytic converter warm-up time. A combi valve is around $200 I think. I'll check for sure in a second. I recommend you take your car to a different mechanic if you've spent that much money throwing parts at your car. The VW specific scan tools such as at the dealer or shops that only work on VW can tell within 5 minutes what the problem is with that system. It sounds like he may be throwing parts at the car, secondly the combi valve if it did fail may have caused the pump to fail as well.
It's around $200 I just checked, each dealer sets their own price though so it may be slightly different where you are.