Personally I think you'll pay minimum $400 but probably more for a good mechanic doing a good job.
Most of the cost is involved in the labour, which is a bit of a bitch seeing as most tranny mechs will jack your car to around head-height and then pull the GBox out from underneath the car. It's a pain in the ass if the engine's in the bay and whether you're changing a clutch or changing the box won't make a huge difference to labour I think.
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